EVALUATION KIT 28V, PWM, Step-Up DC-DC · PDF fileMAX618 28V, PWM, Step-Up DC-DC Converter...

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.For small orders, phone 1-800-835-8769.

General DescriptionThe MAX618 CMOS, PWM, step-up DC-DC convertergenerates output voltages up to 28V and acceptsinputs from +3V to +28V. An internal 2A, 0.3Ω switcheliminates the need for external power MOSFETs whilesupplying output currents up to 500mA or more. APWM control scheme combined with Idle Mode™ oper-ation at light loads minimizes noise and ripple whilemaximizing efficiency over a wide load range. No-loadoperating current is 500µA, which allows efficiency upto 93%.

A fast 250kHz switching frequency allows the use ofsmall surface-mount inductors and capacitors. A shut-down mode extends battery life when the device is notin use. Adaptive slope compensation allows theMAX618 to accommodate a wide range of input andoutput voltages with a simple, single compensationcapacitor.

The MAX618 is available in a thermally enhanced 16-pin QSOP package that is the same size as an industry-standard 8-pin SO but dissipates up to 1W. Anevaluation kit (MAX618EVKIT) is available to helpspeed designs.

ApplicationsAutomotive-Powered DC-DC Converters

Industrial +24V and +28V Systems

LCD Displays

Palmtop Computers

Features♦ Adjustable Output Voltage Up to +28V

♦ Up to 93% Efficiency

♦ Wide Input Voltage Range (+3V to +28V)

♦ Up to 500mA Output Current at +12V

♦ 500µA Quiescent Supply Current

♦ 3µA Shutdown Current

♦ 250kHz Switching Frequency

♦ Small 1W 16-Pin QSOP Package

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28V, PWM, Step-Up DC-DC Converter

________________________________________________________________ Maxim Integrated Products 1

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

GND GND

PGND

PGND

PGND

GND

VL

IN

GND

TOP VIEW

MAX618

QSOP

LX

LX

COMP

LX

SHDN

FB

GND

PGND

LX

FBVL

VOUTUP TO 28V

COMP

IN

SHDN

GND

VIN3V TO 28V

MAX618

Typical Operating Circuit

19-1462; Rev 0; 6/99

Pin Configuration

Ordering Information

Idle Mode is a trademark of Maxim Integrated Products.

16 QSOP

PIN-PACKAGETEMP. RANGE

-40°C to +85°CMAX618EEE

PART

EVALUATION KIT

AVAILABLE

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2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS(VIN = +6V, PGND = GND, CVL = 4.7µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.

IN to GND...............................................................-0.3V to +30VLX to GND ..............................................................-0.3V to +30VVL to GND ................................................................-0.3V to +6VSHDN, COMP, FB to GND............................-0.3V to (VL + 0.3V)PGND to GND.....................................................................±0.3VContinuous Power Dissipation (TA = +70°C) (Note 1)

16-Pin QSOP (derate 15mW/°C above +70°C)...................1W

Operating Temperature Range ...........................-40°C to +85°CJunction Temperature ......................................................+150°CStorage Temperature Range .............................-65°C to +150°CLead Temperature (soldering, 10sec) .............................+300°C

Note 1: With part mounted on 0.9 in.2 of copper.

Shutdown Supply Current IIN 3 8 µAVIN = 28V, VFB = 1.6V, SHDN = GND

Maximum Duty Cycle DC 90 95 %

PARAMETER SYMBOL MIN TYP MAX UNITS

VL Output Voltage VVL 2.9 3.05 3.2 V

Supply Current, Full Load IIN 2.5 3.5 mA

Supply Current, Full Load, VLConnected to IN

IIN 5 6.5 mA

VL Load Regulation ∆VVL 25 40 mV

VL Undervoltage Lockout 2.58 2.7 2.8 V

FB Set Voltage VFB 1.47 1.5 1.53 V

FB Input Bias Current IFB 1 50 nA

Supply Current, No Load

Input Voltage VIN 3 28 V

IIN 500 700 µA

Line Regulation ∆VOUT 0.01 0.08 %/V

Load Regulation ∆VOUT 0.2 %

LX Voltage VLX 28 V

LX Switch Current Limit ILXON 1.7 2.2 2.7 A

Idle Mode Current-LimitThreshold

0.25 0.35 0.45 A

LX On-Resistance RLXON 0.3 0.6 Ω

LX Leakage Current ILXOFF 0.02 10 µA

COMP Maximum Output Current ICOMP 100 200 µA

COMP Current vs. FB VoltageTransconductance

0.8 1 mmho

SHDN Input Logic Low VIL 0.8 V

SHDN Input Logic High VIH 2.0 V

Shutdown Input Current 1 µA

Switching Frequency f 200 250 300 kHz

CONDITIONS

VIN = 3V to 6V, VOUT = 12V

VIN = 3.5V or 28V, no load

VOUT = 12V, ILOAD = 10mA to 500mA

VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL,VVL < VIN

VIN = 3V to 5.5V, VFB = 1.4V, SHDN = VL = IN

ILOAD = 0 to 2mA, VFB = 1.6V

Rising edge, 1% hysteresis

PWM mode

VFB = 1.6V

VLX = 28V

FB = GND

VIN = 3V to 28V, VFB = 1.6V, SHDN = VL

∆FB = 0.1V

SHDN = GND or VL

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_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS(VIN = +6V, PGND = GND, CVL = 4.7µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)

100

00.1 1 10 100 1000

EFFICIENCY vs. OUTPUT CURRENT(VOUT = 12V)

20

30

10

MAX

618

toc0

1

OUTPUT CURRENT (mA)

EFFI

CIEN

CY (%

)

40

50

60

70

80

90VIN = 8V

VIN = 5VVIN = 3V

100

00.1 1 10 100 1000

EFFICIENCY vs. OUTPUT CURRENT(VOUT = 28V)

20

30

10

MAX

618

toc0

2

OUTPUT CURRENT (mA)

EFFI

CIEN

CY (%

)

40

50

60

70

80

90VIN = 12V

VIN = 5V

VIN = 3V

Typical Operating Characteristics(Circuit of Figure 1, TA = +25°C.)

Note 2: Specifications to -40°C are guaranteed by design, not production tested.

PARAMETER SYMBOL MIN TYP MAX UNITS

Supply Current, Full Load IIN 4 mA

Supply Current, Full Load,VL Connected to IN

IIN 7.5 mA

Supply Current Shutdown IIN 10 µA

VL Output Voltage VVL 2.85 3.3 V

Supply Current, No Load

Input Voltage VIN 3 28 V

IIN 800 µA

VL Undervoltage Lockout VVL 2.55 2.85 V

FB Set Voltage VFB 1.455 1.545 V

LX Voltage Range VLXON 28 V

LX Switch Current Limit ILXON 1.4 3 A

LX On-Resistance RLXON 0.6 Ω

Switching Frequency f 188 312 kHz

CONDITIONS

Rising edge, 1% hysteresis

VIN = 3.4V to 28V, VFB = 1.4V, SHDN = VL, VL < VIN

VIN = 3V to 5.5, VFB = 1.4V, SHDN = VL = IN

VIN = 28V, VFB = 1.6V, SHDN = GND

PWM mode

VIN = 3.5V or 28V, no load

VIN = 3V to 28V, VFB = 1.6V, SHDN = VL

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4 _______________________________________________________________________________________

0

VOUT(100mV/div)

VLX(10V/div)

IL(1A/div)

MEDIUM-LOAD SWITCHINGWAVEFORMS

MAX618 toc07

VIN = 5V, VOUT = 12V, IOUT = 200mA

2µs/div

0

VOUT(100mV/

div)

VLX(10V/div)

IL(1A/div)

HEAVY-LOAD SWITCHINGWAVEFORMS

MAX618 toc08

VIN = 5V, VOUT = 12V, IOUT = 500mA

2µs/div

3V

6V

VOUT(50mV/div)

VIN(5V/div)

LINE-TRANSIENT RESPONSEMAX618 toc09

IOUT = 200mA, VOUT = 12V

2ms/div

Typical Operating Characteristics (continued)(Circuit of Figure 1, TA = +25°C.)

0

VOUT(200mV/div)

IOUT(100mA/div)

LOAD-TRANSIENT RESPONSEMAX618 toc10

VIN = 5V, VOUT = 12V

5ms/div

5V

12V

0

SHDN(2V/div)

VOUT(2V/div)

SHUTDOWN RESPONSEMAX618 toc11

VIN = 5V, VOUT = 12V, ILOAD = 500mA

500µs/div0

0.4

0.2

0.6

1.2

1.4

1.0

0.8

1.6

2 4 5 6 73 8 9 10 11 12

MAXIMUM OUTPUT CURRENTvs. INPUT VOLTAGE

MAX

618

toc1

2

INPUT VOLTAGE (V)

MAX

IMUM

OUT

PUT

CURR

ENT

(A)

VOUT = 12V

0.40

0.45

0.55

0.50

0.60

0.65

0 105 15 20 25 30

NO-LOAD SUPPLY CURRENTvs. INPUT VOLTAGE

MAX

618

toc0

4

INPUT VOLTAGE (V)

SUPP

LY C

IRRE

NT (m

A)

300

400

350

500

450

550

600

650

700

-50 -10 10-30 30 50 70 90 110

SUPPLY CURRENT vs. TEMPERATURE

MAX

618

toc0

5

TEMPERATURE (°C)

SUPP

LY C

URRE

NT (µ

A)

VIN = 8V

VIN = 5V

VIN = 3V

INCLUDES CAPACITOR LEAKAGE CURRENT0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2 127 17 22 27 32

SHUTDOWN CURRENTvs. SUPPLY VOLTAGE

MAX

618

toc0

6

SUPPLY VOLTAGE (V)

SHUT

DOW

N CU

RREN

T (µ

A)

_______________ Detailed DescriptionThe MAX618 pulse-width modulation (PWM) DC-DCconverter with an internal 28V switch operates in a widerange of DC-DC conversion applications includingboost, SEPIC, and flyback configurations. The MAX618uses fixed-frequency PWM operation and Maxim’s pro-prietary Idle Mode control to optimize efficiency over awide range of loads. It also features a shutdown modeto minimize quiescent current when not in operation.

PWM Control Scheme and Idle Mode Operation

The MAX618 combines continuous-conduction PWMoperation at medium to high loads and Idle Mode oper-ation at light loads to provide high efficiency over awide range of load conditions. The MAX618 controlscheme actively monitors the output current and auto-matically switches between PWM and Idle Mode tooptimize efficiency and load regulation. Figure 2 showsa functional diagram of the MAX618’s control scheme.

The MAX618 normally operates in low-noise, continu-ous-conduction PWM mode, switching at 250kHz. InPWM mode, the internal MOSFET switch turns on witheach clock pulse. It remains on until either the errorcomparator trips or the inductor current reaches the 2Aswitch-current limit. The error comparator compares thefeedback-error signal, current-sense signal, and slope-compensation signal in one circuit block. When theswitch turns off, energy transfers from the inductor to

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_______________________________________________________________________________________________________ 5

Pin Description

PGND

ECB1Q503LLX

FB

1µF

4.7µFVL

COUT

CP

R1

R2

CIND

VOUTIN

L

SHDN

GNDCOMP

UP TO 28V

3V TO 28VVIN

CCOMP

MAX618

VOUT R1 R2 CIND L COUT CP CCOMP

8V 402kΩ 93.1kΩ 150µF 12µH 150µF 220pF 0.082µF12V 715kΩ 100kΩ 100µF 15µH 100µH 56pF 0.1µF28V 574kΩ 32.4kΩ 86µF 39µH 33µF 47pF 0.47µF

Figure 1. Single-Supply Operation

Feedback Input. Connect a resistor-divider network to set VOUT. FB threshold is 1.5V.FB7

LDO Regulator Supply Input. IN accepts inputs up to +28V. Bypass to GND with a 1µF ceramic capacitoras close to pins 10 and 12 as possible.

IN10

Internal 3.1V LDO Regulator Output. Bypass to GND with a 4.7µF capacitor.VL11

Power Ground, source of internal N-channel switchPGND13, 14, 15

Compensation Input. Bypass to GND with the capacitance value shown in Table 2.COMP6

Shutdown Input. A logic low puts the MAX618 in shutdown mode and reduces supply current to 3µA.SHDN must not exceed VL. In shutdown, the output falls to VIN less one diode drop.SHDN5

PIN

Drain of internal N-channel switch. Connect the inductor between IN and LX.LX2, 3, 4

GroundGND1, 8, 9,12, 16

FUNCTIONNAME

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the output capacitor. Output current is limited by the 2AMOSFET current limit and the MAX618’s packagepower-dissipation limit. See the Maximum OutputCurrent section for details.

In Idle Mode, the MAX618 improves light-load efficien-cy by reducing inductor current and skipping cycles toreduce the losses in the internal switch, diode, andinductor. In this mode, a switching cycle initiates onlywhen the error comparator senses that the output volt-age is about to drop out of regulation. When thisoccurs, the NMOS switch turns on and remains on untilthe inductor current exceeds the nominal 350mA IdleMode current limit.

Refer to Table 1 for an estimate of load currents at whichthe MAX618 transitions between PWM and Idle Mode.

Compensation SchemeAlthough the higher loop gain of voltage-controlledarchitectures tends to provide tighter load regulation,current-controlled architectures are generally easier tocompensate over wide input and output voltage

ranges. The MAX618 uses both control schemes in par-allel: the dominant, low-frequency components of theerror signal are tightly regulated with a voltage-controlloop, while a current-control loop improves stability athigher frequencies. Compensation is achieved throughthe selection of the output capacitor (COUT), the inte-grator capacitor (CCOMP), and the pole capacitor (CP)from FB to GND. CP cancels the zero formed by COUTand its ESR. Refer to the Capacitor Selection section forguidance on selecting these capacitors.

VL Low-Dropout RegulatorThe MAX618 contains a 3.1V low-dropout linear regula-tor to power internal circuitry. The regulator’s input is INand its output is VL. The IN to VL dropout voltage is100mV, so that when IN is less than 3.2V, VL is typically100mV below IN. The MAX618 still operates when theLDO is in dropout, as long as VL remains above the2.7V undervoltage lockout. Bypass VL with a 4.7µFceramic capacitor placed as close to the VL and GNDpins as possible.


6 _______________________________________________________________________________________

SHDN

MAX618

IDLE MODECURRENT LIMIT

PWMCURRENT LIMIT

ERRORCOMPARATOR

250kHzOSCILLATOR

SLOPECOMPENSATION

LINEARREGULATOR

CURRENT-SENSE

CIRCUIT

PGND IN

LX OUTR

14R

VL

FB

COMP

IN

VL

NMOS

REFERENCE

INTEGRATOR

GND

SHUTDOWN

PWMLOGIC

THERMALSHUTDOWN

Figure 2. Functional Diagram

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_______________________________________________________________________________________ 7

45

67

89

1011

1213

1415

1617

1819

2021

2223

2425

2627

283

0.20

0.20

0.18

0.15

0.12

0.10

0.09

0.08

0.07

0.06

0.05

0.04

0.04

0.04

0.03

0.03

0.03

0.03

0.03

0.02

0.02

0.02

0.02

0.02

0.02

40.

180.

210.

200.

170.

150.

130.

120.

100.

090.

080.

070.

070.

060.

050.

050.

040.

040.

040.

030.

030.

030.

030.

030.

035

0.16

0.20

0.21

0.19

0.17

0.16

0.14

0.13

0.11

0.10

0.09

0.09

0.08

0.07

0.07

0.06

0.06

0.05

0.05

0.04

0.04

0.04

0.04

60.

150.

200.

210.

200.

190.

180.

160.

150.

130.

120.

110.

100.

100.

090.

080.

080.

070.

070.

060.

060.

050.

057

0.17

0.19

0.21

0.21

0.20

0.19

0.17

0.16

0.15

0.14

0.13

0.12

0.11

0.10

0.10

0.09

0.08

0.08

0.07

0.07

0.07

80.

190.

180.

200.

210.

200.

200.

190.

170.

160.

150.

140.

130.

130.

120.

110.

100.

100.

090.

090.

089

0.20

0.17

0.20

0.21

0.21

0.20

0.19

0.18

0.18

0.17

0.16

0.15

0.14

0.13

0.12

0.12

0.11

0.10

0.10

100.

210.

160.

190.

200.

210.

210.

200.

190.

180.

170.

170.

160.

150.

140.

130.

130.

120.

1111

0.22

0.15

0.19

0.20

0.21

0.21

0.20

0.20

0.19

0.18

0.17

0.17

0.16

0.15

0.14

0.14

0.13

120.

230.

150.

180.

200.

210.

210.

210.

200.

200.

190.

180.

180.

170.

160.

150.

1513

0.24

0.16

0.17

0.19

0.20

0.21

0.21

0.20

0.20

0.19

0.19

0.18

0.17

0.17

0.16

140.

250.

170.

170.

190.

200.

210.

210.

210.

200.

200.

190.

190.

180.

1715

0.25

0.18

0.16

0.18

0.20

0.20

0.21

0.21

0.21

0.20

0.20

0.19

0.19

160.

260.

190.

160.

180.

190.

200.

210.

210.

210.

200.

200.

2017

0.26

0.20

0.15

0.17

0.19

0.20

0.20

0.21

0.21

0.21

0.20

180.

270.

200.

150.

170.

190.

200.

200.

210.

210.

2119

0.27

0.21

0.16

0.17

0.18

0.19

0.20

0.21

0.21

200.

270.

210.

170.

160.

180.

190.

200.

2021

0.28

0.22

0.17

0.16

0.18

0.19

0.20

220.

280.

220.

180.

150.

170.

1923

0.28

0.23

0.18

0.15

0.17

240.

280.

230.

190.

1525

0.29

0.24

0.19

260.

290.

2427

0.29

VO

UT

VIN

Tab

le 1

. PW

M/Id

le-M

od

e T

ran

siti

on

Lo

ad C

urr

ent

(IO

UT

in A

mp

s) v

s. In

pu

t an

d O

utp

ut

Vo

ltag

e

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8 _______________________________________________________________________________________

GND

LX

COUT

L

PGND

R2

R1

CP

VL

COMP

CCOMP

IN

SHDN

UP TO 28V

VINDUP TO 28V

CIND

IN OUT

MAX618

2.7V TO 5.5V

4.7µF

1µF

FB

LX

CIND

COUT

L

PGND

GND

R1

VL

COMPCCOMP

IN

SHDN

UP TO 28V

VINDUP TO 28V

OUT

MAX618

IN3V TO 28V

4.7µF

1µF

R2CP

FB

Figure 3. Dual-Supply Operation (VIN = 2.7V to 5.5V) Figure 4. Dual-Supply Operation (VIN = 3V to 28V)

Table 2. Input Configurations

VL can be overdriven by an external supply between2.7V and 5.5V. In systems with +3.3V or +5V logicpower supplies available, improve efficiency by power-ing VL and VIN directly from the logic supply as shownin Figure 3.

Operating ConfigurationsThe MAX618 can be connected in one of three configura-tions described in Table 2 and shown in Figures 1, 3, and4. The VL linear regulator allows operation from a singlesupply between +3V and +28V as shown in Figure 1.

The circuit in Figure 3 allows a logic supply to powerthe MAX618 while using a separate source for DC-DCconversion power (inductor voltage). The logic supply(between 2.7V and 5.5V) connects to VL and IN. VL =IN; voltages of 3.3V or more improve efficiency by pro-viding greater gate drive for the internal MOSFET.

The circuit in Figure 4 allows separate supplies topower IN and the inductor voltage. It differs from theconnection in Figure 3 in that the MAX618 chip supplyis not limited to 5.5V.

CIRCUIT

Figure 1Input voltage connectsto IN and inductor.

CONNECTIONVIN

RANGE

3V to VOUT(up to 28V)

VIN

INDUCTORVOLTAGE

BENEFITS/COMMENTS

• Single-supply operation. • SHDN must be connected to or pulled up to VL. On/off

control requires an open-drain or open-collector connectionto SHDN.

Figure 3

Figure 40 to VOUT

(up to 28V)

0 to VOUT(up to 28V)

• Increased efficiency.• SHDN can be driven by logic powered from the supply con-

nected to IN and VL, or can be connected to or pulled up toVL.

• Input power source (inductor voltage) is separate from theMAX618’s bias (VIN = VL) and can be less than or greaterthan VIN.

• Input power source (inductor voltage) is separate from theMAX618’s bias (VIN) and can be less than or greater thanVIN.

• SHDN must be connected to or pulled up to VL. On/off control requires an open-drain or open-collector connectionto SHDN.

IN and inductor volt-age supplied by sepa-rate sources.

IN and VL connecttogether. Inductor volt-age supplied by a separate source.

2.7V to 5.5V

3V to 28V

Shutdown ModeIn shutdown mode (SHDN = 0), the MAX618’s feed-back and control circuit, reference, and internal biasingcircuitry turn off and reduce the IN supply current to3µA (10µA max). When in shutdown, a current pathremains from the input to the output through the exter-nal inductor and diode. Consequently, the output fallsto VIN less one diode drop in shutdown.

SHDN may not exceed VL. For always-on operation,connect SHDN to VL. To add on/off control to the circuitof Figure 1 or 4, pull SHDN to VL with a resistor (10kΩto 100kΩ) and drive SHDN with an open-drain logicgate or switch as shown in Figure 5. Alternatively, thecircuit of Figure 3 allows direct SHDN drive by anylogic-level gate powered from the same supply thatpowers VL and IN, as shown in Figure 6.

__________________Design ProcedureThe MAX618 operates in a number of DC-DC converterconfigurations including step-up, SEPIC, and flyback.The following design discussion is limited to step-upconverters.

Setting the Output VoltageTwo external resistors (R1 and R2) set the output volt-age. First, select a value for R2 between 10kΩ and200kΩ. Calculate R1 with:

where VFB is 1.5V.

Determining the Inductor ValueThe MAX618’s high switching frequency allows the useof a small value inductor. The recommended inductorvalue is proportional to the output voltage and is givenby the following:

After solving for the above equation, round down asnecessary to select a standard inductor value.

When selecting an inductor, choose one rated to250kHz, with a saturation current exceeding the peakinductor current, and with a DC resistance under200mΩ. Ferrite core or equivalent inductors are gener-ally appropriate (see MAX618 EV kit data sheet).Calculate the peak inductor current with the followingequation:

Note that the peak inductor current is internally limitedto 2A.

Diode SelectionThe MAX618’s high switching frequency demands ahigh-speed rectifier. Schottky diodes are preferred formost applications because of their fast recovery timeand low forward voltage. Make sure that the diode’speak current rating exceeds the 2A peak switch cur-rent, and that its breakdown voltage exceeds the out-put voltage.

I IV

V2 s

V

L

V V

VLX(PEAK) OUT

OUT

IN

IN OUT IN

OUT= +

⎛

⎝⎜

⎞

⎠⎟

−( )⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

µ

LVOUT=⋅7 105

R RVVOUT

FB1 2 1= −

⎛

⎝⎜

⎞

⎠⎟

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_______________________________________________________________________________________ 9

MAX618VL

100k

ON/OFF CONTROL

OPEN-DRAINLOGIC

SHDN

MAX618IN

VL

SYSTEM LOGICON/OFF

CONTROL

SHDN

SYSTEMLOGIC SUPPLY

Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4 Figure 6. Adding On/Off Control to Circuit of Figure 3

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10 ______________________________________________________________________________________

Maximum Output CurrentThe MAX618’s 2.2A LX current limit determines theoutput power that can be supplied for most applica-tions. In some cases, particularly when the input volt-age is low, output power is sometimes restricted bypackage dissipation limits. The MAX618 is protectedby a thermal shutdown circuit that turns off the switchwhen the die temperature exceeds +150°C. When thedevice cools by 10°C, the switch is enabled again.Table 3 details output current with a variety of input andoutput voltages. Each listing in Table 3 is either the limitset by an LX current limit or by package dissipation at+85°C ambient, whichever is lower. The values in Table3 assume a 40mΩ inductor resistance.

Capacitor SelectionInput Capacitors

The input bypass capacitor, CIND, reduces the inputripple created by the boost configuration. High-imped-ance sources require high CIND values. However, 68µFis generally adequate for input currents up to 2A. LowESR capacitors are recommended because they willdecrease the ripple created on the input and improveefficiency. Capacitors with ESR below 0.3Ω are gener-ally appropriate.

In addition to the input bypass capacitor, bypass INwith a 1µF ceramic capacitor placed as close to the INand GND pins as possible. Bypass VL with a 4.7µFceramic capacitor placed as close to the VL and GNDpins as possible.

Output CapacitorUse Table 4 to find the minimum output capacitancenecessary to ensure stable operation. In addition,choose an output capacitor with low ESR to reduce theoutput ripple. The dominant component of output rippleis the product of the peak-to-peak inductor ripple cur-rent and the ESR of the output capacitor. ESR below50mΩ generates acceptable levels of output ripple formost applications.

Integrator CapacitorThe compensation capacitor (CCOMP) sets the domi-nant pole in the MAX618’s transfer function. The propercompensation capacitance depends upon outputcapacitance. Table 5 shows the capacitance value

needed for the output capacitances specified in Table4. However, if a different output capacitor is used (e.g.,a standard value), then recalculate the value of capaci-tance needed for the integrator capacitor with the fol-lowing formula:

Pole Compensation CapacitorThe pole capacitor (CP) cancels the unwanted zerointroduced by COUT’s ESR, and thereby ensures stabil-ity in PWM operation. The exact value of the polecapacitor is not critical, but it should be near the valuecalculated by the following equation:

where RESR is COUT’s ESR.

Layout ConsiderationsProper PC board layout is essential due to high currentlevels and fast switching waveforms that radiate noise.Use the MAX618 evaluation kit or equivalent PC layoutto perform initial prototyping. Breadboards, wire-wrap,and proto-boards are not recommended when proto-typing switching regulators.

It is important to connect the GND pin, the inputbypass capacitor ground lead, and the output filtercapacitor ground lead to a single point to minimizeground noise and improve regulation. Also, minimizelead lengths to reduce stray capacitance, trace resis-tance, and radiated noise, with preference given to thefeedback circuit, the ground circuit, and LX. Place thefeedback resistors as close to the FB pin as possible.Place a 1µF input bypass capacitor as close as possi-ble to IN and GND.

Refer to the MAX618 evaluation kit for an example ofproper board layout.

CR C (R2 R2)

R1 R2 P

ESR OUT

=

+⋅⋅

CC Table C

C TableCOMPCOMP OUT

OUT=

⋅( )( )

54

MA

X6

18


______________________________________________________________________________________ 11

45

67

89

1011

1213

1415

1617

1819

2021

2223

2425

2627

283

0.77

0.59

0.49

0.41

0.34

0.29

0.25

0.22

0.20

0.18

0.17

0.15

0.14

0.13

0.12

0.12

0.11

0.10

0.10

0.09

0.09

0.08

0.08

0.08

0.07

40.

960.

760.

640.

560.

490.

430.

380.

340.

310.

280.

260.

240.

220.

210.

190.

180.

170.

160.

160.

150.

140.

140.

130.

125

1.09

0.89

0.76

0.67

0.60

0.54

0.50

0.45

0.41

0.37

0.34

0.32

0.30

0.28

0.26

0.25

0.23

0.22

0.21

0.20

0.19

0.18

0.18

61.

180.

990.

850.

760.

680.

630.

580.

540.

500.

460.

420.

390.

370.

340.

320.

310.

290.

280.

260.

250.

240.

237

1.26

1.07

0.93

0.83

0.76

0.70

0.65

0.60

0.57

0.53

0.50

0.46

0.43

0.41

0.38

0.36

0.35

0.33

0.31

0.30

0.29

81.

321.

131.

000.

900.

820.

760.

710.

660.

620.

590.

560.

530.

500.

470.

440.

420.

400.

380.

360.

359

1.37

1.19

1.06

0.96

0.88

0.81

0.76

0.71

0.67

0.64

0.61

0.58

0.55

0.53

0.50

0.47

0.45

0.43

0.41

101.

411.

241.

111.

010.

930.

860.

810.

760.

720.

680.

650.

620.

590.

570.

550.

520.

500.

4711

1.44

1.28

1.15

1.05

0.97

0.91

0.85

0.80

0.76

0.72

0.69

0.66

0.63

0.61

0.58

0.56

0.54

121.

471.

311.

191.

101.

020.

950.

890.

840.

800.

760.

730.

700.

670.

640.

620.

6013

1.49

1.34

1.23

1.13

1.05

0.99

0.93

0.88

0.83

0.80

0.76

0.73

0.70

0.67

0.65

141.

521.

371.

261.

161.

091.

020.

960.

910.

870.

830.

790.

760.

730.

7115

1.53

1.40

1.29

1.19

1.12

1.05

0.99

0.94

0.90

0.86

0.82

0.79

0.76

161.

551.

421.

311.

221.

141.

081.

020.

970.

930.

890.

850.

8217

1.57

1.44

1.33

1.25

1.17

1.11

1.05

1.00

0.95

0.91

0.88

181.

581.

461.

361.

271.

201.

131.

071.

020.

980.

9419

1.59

1.47

1.37

1.29

1.22

1.15

1.10

1.05

1.00

201.

601.

491.

391.

311.

241.

181.

121.

0721

1.61

1.50

1.41

1.33

1.26

1.20

1.14

221.

621.

511.

421.

351.

281.

2223

1.63

1.53

1.44

1.36

1.29

241.

641.

541.

451.

3825

1.64

1.55

1.46

261.

651.

5627

1.66

VIN

VO

UT

Tab

le 3

. Typ

ical

Ou

tpu

t C

urr

ent

vs. I

np

ut

and

Ou

tpu

t V

olt

ages

MA

X6

18


12 ______________________________________________________________________________________

45

67

89

1011

1213

1415

1617

1819

2021

2223

2425

2627

283

173

128

100

8065

5446

4035

3128

2523

2119

1817

1515

1413

1212

1110

415

111

896

8068

5951

4539

3532

2927

2423

2120

1817

1615

1514

135

132

107

9077

6759

5246

4137

3431

2926

2523

2120

1918

1716

156

117

9783

7264

5751

4642

3835

3230

2826

2423

2120

1918

177

104

8977

6861

5550

4542

3935

3330

2826

2523

2221

2019

894

8272

6458

5248

4441

3835

3331

2927

2524

2221

209

8676

6761

5550

4642

3937

3432

3029

2725

2423

2110

7970

6357

5248

4441

3836

3432

3028

2725

2423

1173

6659

5450

4643

4037

3533

3129

2826

2524

1268

6256

5147

4441

3836

3432

3029

2726

2513

6458

5349

4542

3937

3533

3129

2827

2514

6055

5047

4340

3836

3432

3029

2726

1556

5248

4442

3937

3533

3129

2827

1653

4946

4340

3735

3332

3029

2717

5047

4441

3836

3432

3129

2818

4845

4239

3735

3331

3028

1946

4340

3836

3432

3029

2043

4138

3634

3331

2921

4239

3735

3332

3022

4038

3634

3231

2338

3634

3331

2437

3533

3225

3534

3226

3433

2733

VIN

VO

UT

Tab

le 4

. Min

imu

m C

OU

Tfo

r S

tab

ility

(µ

F)

MA

X6

18


______________________________________________________________________________________ 13

45

67

89

1011

1213

1415

1617

1819

2021

2223

2425

2627

283

4046

5464

7383

9410

511

813

014

315

717

218

720

321

923

625

327

129

030

932

934

937

039

14

4245

5158

6674

8291

100

109

119

130

141

152

164

176

188

201

214

228

242

257

272

287

543

4549

5460

6775

8188

9610

311

112

012

813

714

715

616

617

618

719

720

922

06

4445

4852

5762

6874

8086

9299

105

112

119

127

134

142

150

159

167

176

745

4547

5054

5863

6874

7985

9095

101

107

113

119

125

132

139

146

846

4547

4952

5660

6468

7378

8388

9398

103

108

113

119

124

946

4647

4851

5457

6164

6873

7782

8691

9599

104

109

1047

4646

4850

5255

5861

6569

7277

8185

8993

9711

4746

4648

4951

5456

5962

6569

7276

8084

8812

4847

4747

4950

5255

5760

6366

6972

7579

1348

4747

4748

5052

5456

5861

6366

6972

1449

4747

4748

4951

5355

5759

6164

6615

4947

4747

4849

5052

5355

5759

6216

4948

4747

4849

5051

5354

5658

1749

4847

4748

4849

5152

5355

1850

4847

4748

4849

5051

5319

5048

4747

4848

4950

5120

5048

4847

4848

4949

2150

4948

4748

4848

2250

4948

4848

4823

5049

4848

4824

5149

4848

2551

4948

2651

4927

51

VO

UT

VIN

Tab

le 5

. Min

imu

m C

CO

MP

for

Sta

bili

ty (

nF

)

MA

X6

18


14 ______________________________________________________________________________________

Package Information

QS

OP

.EP

S

TRANSISTOR COUNT: 1794

___________________Chip Information

EVALUATION KIT 28V, PWM, Step-Up DC-DC · PDF fileMAX618 28V, PWM, Step-Up DC-DC Converter...

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