MAXIM MAX618 User Manual

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General Description

The MAX618 CMOS, PWM, step-up DC-DC converter
generates output voltages up to 28V and accepts
inputs from +3V to +28V. An internal 2A, 0.3Ω switch
eliminates the need for external power MOSFETs while
supplying output currents up to 500mA or more. A
PWM control scheme combined with Idle Mode™ oper­ation at light loads minimizes noise and ripple while
maximizing efficiency over a wide load range. No-load
operating current is 500µA, which allows efficiency up
to 93%.

A fast 250kHz switching frequency allows the use of
small surface-mount inductors and capacitors. A shut­down mode extends battery life when the device is not
in use. Adaptive slope compensation allows the
MAX618 to accommodate a wide range of input and
output voltages with a simple, single compensation
capacitor.

The MAX618 is available in a thermally enhanced 16­pin QSOP package that is the same size as an industry­standard 8-pin SO but dissipates up to 1W. An
evaluation kit (MAX618EVKIT) is available to help
speed designs.

Applications

Automotive-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

MAX618

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

________________________________________________________________ Maxim Integrated Products 1

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

V

TOP VIEW



16

15

PGND

14

PGND

13

PGND

12

GND

VL

11

10

IN

9

GND

1

GND GND

LX

2

LX

3

MAX618

4

LX

SHDN

5

COMP

6

FB

7

GND

8

QSOP

3V TO 28V

IN

IN

MAX618

SHDN

VL

COMP

LX

PGND

FB

GND

V

OUT

UP TO 28V

MAX618

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

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V

IN

= +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 functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

IN to GND ...............................................................-0.3V to +30V

LX to GND ..............................................................-0.3V to +30V

VL to GND ................................................................-0.3V to +6V

SHDN, COMP, FB to GND ............................-0.3V to (VL + 0.3V)

PGND to GND.....................................................................±0.3V

Continuous Power Dissipation (T

A

= +70°C) (Note 1)

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

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10sec) .............................+300°C

Note 1: With part mounted on 0.9 in.

2

of copper.

Shutdown Supply Current I

IN

38

µA

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

Maximum Duty Cycle DC 90 95 %

PARAMETER SYMBOL MIN TYP MAX UNITS

VL Output Voltage V

VL

2.9 3.05 3.2

V

Supply Current, Full Load I

IN

2.5 3.5

mA

Supply Current, Full Load, VL
Connected to IN

I

IN

5 6.5

mA

VL Load Regulation ∆V

VL

25 40

mV

VL Undervoltage Lockout

2.58 2.7 2.8

V

FB Set Voltage V

FB

1.47 1.5 1.53

V

FB Input Bias Current I

FB

150

nA

Supply Current, No Load

Input Voltage V

IN

328

V

I

IN

500 700

µA

Line Regulation ∆V

OUT

0.01 0.08

%/V

Load Regulation ∆V

OUT

0.2

%

LX Voltage V

LX

28 V

LX Switch Current Limit I

LXON

1.7 2.2 2.7

A

Idle Mode Current-Limit
Threshold

0.25 0.35 0.45

A

LX On-Resistance R

LXON

0.3 0.6 Ω

LX Leakage Current I

LXOFF

0.02 10 µA

COMP Maximum Output Current I

COMP

100 200 µA

COMP Current vs. FB Voltage
Transconductance

0.8 1 mmho

SHDN Input Logic Low

V

IL

0.8 V

SHDN Input Logic High

V

IH

2.0 V

Shutdown Input Current 1 µA

Switching Frequency f 200 250 300 kHz

CONDITIONS

VIN= 3V to 6V, V

OUT

= 12V

VIN= 3.5V or 28V, no load

V

OUT

= 12V, I

LOAD

= 10mA to 500mA

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

VL

< V

IN

V

IN

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

I

LOAD

= 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

MAX618

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

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS

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

100

0

0.1 1 10 100 1000

EFFICIENCY vs. OUTPUT CURRENT

(V

OUT

= 12V)

20

30

10

MAX618 toc01

OUTPUT CURRENT (mA)

EFFICIENCY (%)

40

50

60

70

80

90

VIN = 8V

VIN = 5V

VIN = 3V

100

0

0.1 1 10 100 1000

EFFICIENCY vs. OUTPUT CURRENT

(V

OUT

= 28V)

20

30

10

MAX618 toc02

OUTPUT CURRENT (mA)

EFFICIENCY (%)

40

50

60

70

80

90

VIN = 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 I

IN

4

mA

Supply Current, Full Load,
VL Connected to IN

I

IN

7.5

mA

Supply Current Shutdown I

IN

10

µA

VL Output Voltage V

VL

2.85 3.3

V

Supply Current, No Load

Input Voltage V

IN

328

V

I

IN

800

µA

VL Undervoltage Lockout V

VL

2.55 2.85

V

FB Set Voltage V

FB

1.455 1.545

V

LX Voltage Range V

LXON

28

V

LX Switch Current Limit I

LXON

1.4 3

A

LX On-Resistance R

LXON

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 < V

IN

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

MAX618

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

4 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA= +25°C.)

0

V

OUT

(200mV/div)

I

OUT

(100mA/div)

LOAD-TRANSIENT RESPONSE

MAX618 toc10

VIN = 5V, V

OUT

= 12V

5ms/div

5V

12V

0

SHDN

(2V/div)

V

OUT

(2V/div)

SHUTDOWN RESPONSE

MAX618 toc11

VIN = 5V, V

OUT

= 12V, I

LOAD

= 500mA

500µs/div

0

0.4

0.2

0.6

1.2

1.4

1.0

0.8

1.6

2 45673 8 9 101112

MAXIMUM OUTPUT CURRENT

vs. INPUT VOLTAGE

MAX618 toc12

INPUT VOLTAGE (V)

MAXIMUM OUTPUT CURRENT (A)

V

OUT

= 12V

0.40

0.45

0.55

0.50

0.60

0.65

0105 15202530

NO-LOAD SUPPLY CURRENT

vs. INPUT VOLTAGE

MAX618 toc04

INPUT VOLTAGE (V)

SUPPLY CIRRENT (mA)

300

400

350

500

450

550

600

650

700

-50 -10 10-30 30507090110

SUPPLY CURRENT vs. TEMPERATURE

MAX618 toc05

TEMPERATURE (°C)

SUPPLY CURRENT (µA)

VIN = 8V

VIN = 5V

VIN = 3V

INCLUDES CAPACITOR LEAKAGE CURRENT

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2127 17222732

SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE

MAX618 toc06

SUPPLY VOLTAGE (V)

SHUTDOWN CURRENT (µA)

MEDIUM-LOAD SWITCHING

I

L

(1A/div)

V

LX

(10V/div)

V

OUT

(100mV/div)

VIN = 5V, V

OUT

WAVEFORMS

2µs/div

= 12V, I

OUT

= 200mA

MAX618 toc07

HEAVY-LOAD SWITCHING

WAVEFORMS

I

L

(1A/div)

0

V

LX

(10V/div)

V

OUT

(100mV/

div)

2µs/div

VIN = 5V, V

OUT

= 12V, I

OUT

= 500mA

MAX618 toc08

V

(50mV/div)

0

(5V/div)

LINE-TRANSIENT RESPONSE

OUT

V

IN

I

= 200mA, V

MAX618 toc09

6V

3V

2ms/div

= 12V

_______________ Detailed Description

The MAX618 pulse-width modulation (PWM) DC-DC
converter with an internal 28V switch operates in a wide
range of DC-DC conversion applications including
boost, SEPIC, and flyback configurations. The MAX618
uses fixed-frequency PWM operation and Maxim’s pro­prietary Idle Mode control to optimize efficiency over a
wide range of loads. It also features a shutdown mode
to minimize quiescent current when not in operation.

PWM Control Scheme and

Idle Mode Operation

The MAX618 combines continuous-conduction PWM
operation at medium to high loads and Idle Mode oper­ation at light loads to provide high efficiency over a
wide range of load conditions. The MAX618 control
scheme actively monitors the output current and auto­matically switches between PWM and Idle Mode to
optimize efficiency and load regulation. Figure 2 shows
a functional diagram of the MAX618’s control scheme.

The MAX618 normally operates in low-noise, continu­ous-conduction PWM mode, switching at 250kHz. In
PWM mode, the internal MOSFET switch turns on with
each clock pulse. It remains on until either the error
comparator trips or the inductor current reaches the 2A
switch-current limit. The error comparator compares the
feedback-error signal, current-sense signal, and slope­compensation signal in one circuit block. When the
switch turns off, energy transfers from the inductor to

MAX618

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

_______________________________________________________________________________________________________ 5

Pin Description

Figure 1. Single-Supply Operation

Feedback Input. Connect a resistor-divider network to set V

OUT

. FB threshold is 1.5V.FB7

LDO Regulator Supply Input. IN accepts inputs up to +28V. Bypass to GND with a 1µF ceramic capacitor
as 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 V

IN

less one diode drop.

SHDN

5

PIN

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

GroundGND

1, 8, 9,

12, 16

FUNCTIONNAME

IN

SHDN

V

L

COMP

L

MAX618

PGND

IND

3V TO 28V

V

IN

C

IND

1µF

4.7µF

C

COMP

V

R1 R2 C

OUT

8V 402kΩ 93.1kΩ 150µF12µH 150µF 220pF 0.082µF
12V 715kΩ 100kΩ 100µF15µH 100µH 56pF 0.1µF
28V 574kΩ 32.4kΩ 86µF39µH33µF 47pF 0.47µF

LX

FB

GND

LC

ECB1Q503L

C

P

OUT

C

OUT

CPC

V

OUT

UP TO 28V

R1

R2

COMP

MAX618

the output capacitor. Output current is limited by the 2A
MOSFET current limit and the MAX618’s package
power-dissipation limit. See the Maximum Output
Current section for details.

In Idle Mode, the MAX618 improves light-load efficien­cy by reducing inductor current and skipping cycles to
reduce the losses in the internal switch, diode, and
inductor. In this mode, a switching cycle initiates only
when the error comparator senses that the output volt­age is about to drop out of regulation. When this
occurs, the NMOS switch turns on and remains on until
the inductor current exceeds the nominal 350mA Idle
Mode current limit.

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

Compensation Scheme

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

ranges. The MAX618 uses both control schemes in par­allel: the dominant, low-frequency components of the
error signal are tightly regulated with a voltage-control
loop, while a current-control loop improves stability at
higher frequencies. Compensation is achieved through
the selection of the output capacitor (C

OUT

), the inte-

grator capacitor (C

COMP

), and the pole capacitor (CP)

from FB to GND. CPcancels the zero formed by C

OUT

and its ESR. Refer to the Capacitor Selection section for
guidance on selecting these capacitors.

VL Low-Dropout Regulator

The MAX618 contains a 3.1V low-dropout linear regula­tor to power internal circuitry. The regulator’s input is IN
and its output is VL. The IN to VL dropout voltage is
100mV, so that when IN is less than 3.2V, VL is typically
100mV below IN. The MAX618 still operates when the
LDO is in dropout, as long as VL remains above the

2.7V undervoltage lockout. Bypass VL with a 4.7µF
ceramic capacitor placed as close to the VL and GND
pins as possible.

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

6 _______________________________________________________________________________________

Figure 2. Functional Diagram

IDLE MODE
CURRENT LIMIT

GND

SHDN

SHUTDOWN

ERROR

COMPARATOR

SLOPE

COMPENSATION

THERMAL

SHUTDOWN

CURRENT LIMIT

250kHz

OSCILLATOR

REFERENCE

PWM

PWM

LOGIC

MAX618

CURRENT-

SENSE

CIRCUIT

VL

NMOS

R

14R

INTEGRATOR

LINEAR

REGULATOR

IN

PGND

LX OUT

FB

COMP

IN

VL

MAX618

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

_______________________________________________________________________________________ 7

Table 1. PWM/Idle-Mode Transition Load Current (I

OUT

in Amps) vs. Input and Output Voltage

OUT

V

45678910111213141516171819202122232425262728

IN

3 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

4 0.18 0.21 0.20 0.17 0.15 0.13 0.12 0.10 0.09 0.08 0.07 0.07 0.06 0.05 0.05 0.04 0.04 0.04 0.03 0.03 0.03 0.03 0.03 0.03

5 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

6 0.15 0.20 0.21 0.20 0.19 0.18 0.16 0.15 0.13 0.12 0.11 0.10 0.10 0.09 0.08 0.08 0.07 0.07 0.06 0.06 0.05 0.05

7 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

8 0.19 0.18 0.20 0.21 0.20 0.20 0.19 0.17 0.16 0.15 0.14 0.13 0.13 0.12 0.11 0.10 0.10 0.09 0.09 0.08

9 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

10 0.21 0.16 0.19 0.20 0.21 0.21 0.20 0.19 0.18 0.17 0.17 0.16 0.15 0.14 0.13 0.13 0.12 0.11

11 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

12 0.23 0.15 0.18 0.20 0.21 0.21 0.21 0.20 0.20 0.19 0.18 0.18 0.17 0.16 0.15 0.15

13 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

14 0.25 0.17 0.17 0.19 0.20 0.21 0.21 0.21 0.20 0.20 0.19 0.19 0.18 0.17

15 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

16 0.26 0.19 0.16 0.18 0.19 0.20 0.21 0.21 0.21 0.20 0.20 0.20

17 0.26 0.20 0.15 0.17 0.19 0.20 0.20 0.21 0.21 0.21 0.20

18 0.27 0.20 0.15 0.17 0.19 0.20 0.20 0.21 0.21 0.21

V

19 0.27 0.21 0.16 0.17 0.18 0.19 0.20 0.21 0.21

20 0.27 0.21 0.17 0.16 0.18 0.19 0.20 0.20

21 0.28 0.22 0.17 0.16 0.18 0.19 0.20

22 0.28 0.22 0.18 0.15 0.17 0.19

23 0.28 0.23 0.18 0.15 0.17

24 0.28 0.23 0.19 0.15

25 0.29 0.24 0.19

26 0.29 0.24

27 0.29

MAX618

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

8 _______________________________________________________________________________________

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

Figure 4. Dual-Supply Operation (V

IN

= 3V to 28V)

Table 2. Input Configurations

VL can be overdriven by an external supply between

2.7V and 5.5V. In systems with +3.3V or +5V logic
power supplies available, improve efficiency by power­ing VL and V

IN

directly from the logic supply as shown

in Figure 3.

Operating Configurations

The MAX618 can be connected in one of three configura­tions described in Table 2 and shown in Figures 1, 3, and

4. The VL linear regulator allows operation from a single
supply between +3V and +28V as shown in Figure 1.

The circuit in Figure 3 allows a logic supply to power
the MAX618 while using a separate source for DC-DC
conversion 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 pro­viding greater gate drive for the internal MOSFET.

The circuit in Figure 4 allows separate supplies to
power IN and the inductor voltage. It differs from the
connection in Figure 3 in that the MAX618 chip supply
is not limited to 5.5V.

CIRCUIT

Figure 1

Input voltage connects
to IN and inductor.

CONNECTION

V

IN

RANGE

3V to V

OUT

(up to 28V)

V

IN

INDUCTOR

VOLTAGE

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 connection
to SHDN.

Figure 3

Figure 4

0 to V

OUT

(up to 28V)

0 to V

OUT

(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 to
VL.

• Input power source (inductor voltage) is separate from the
MAX618’s bias (V

IN

= VL) and can be less than or greater

than VIN.

• Input power source (inductor voltage) is separate from the

MAX618’s bias (V

IN

) and can be less than or greater than

V

IN

.

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

IN and inductor volt­age supplied by sepa­rate sources.

IN and VL connect
together. Inductor volt­age supplied by a
separate source.

2.7V to 5.5V

3V to 28V

V

2.7V TO 5.5V

1µF

IND

UP TO 28V

C

IN

4.7µF

C

COMP

IND

IN

SHDN

VL

COMP

L

MAX618

PGND

GND

LX

C

OUT

FB

C

P

OUT

UP TO 28V

R1

R2

V

IND

UP TO 28V

IN

3V TO 28V

4.7µF

C

COMP

C

1µF

IND

IN

SHDN

VL

COMP

L

LX

MAX618

PGND

FB

GND

C

OUT

C

P

OUT

UP TO 28V

R1

R2

Shutdown Mode

In shutdown mode (SHDN = 0), the MAX618’s feed­back and control circuit, reference, and internal biasing
circuitry turn off and reduce the IN supply current to
3µA (10µA max). When in shutdown, a current path
remains from the input to the output through the exter­nal inductor and diode. Consequently, the output falls
to V

IN

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 circuit
of Figure 1 or 4, pull SHDN to VL with a resistor (10kΩ
to 100kΩ) and drive SHDN with an open-drain logic
gate or switch as shown in Figure 5. Alternatively, the
circuit of Figure 3 allows direct SHDN drive by any
logic-level gate powered from the same supply that
powers VL and IN, as shown in Figure 6.

__________________Design Procedure

The MAX618 operates in a number of DC-DC converter
configurations including step-up, SEPIC, and flyback.
The following design discussion is limited to step-up
converters.

Setting the Output Voltage

Two external resistors (R1 and R2) set the output volt­age. First, select a value for R2 between 10kΩ and
200kΩ. Calculate R1 with:

where VFBis 1.5V.

Determining the Inductor Value

The MAX618’s high switching frequency allows the use
of a small value inductor. The recommended inductor
value is proportional to the output voltage and is given
by the following:

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

When selecting an inductor, choose one rated to
250kHz, with a saturation current exceeding the peak
inductor current, and with a DC resistance under
200mΩ. Ferrite core or equivalent inductors are gener­ally appropriate (see MAX618 EV kit data sheet).
Calculate the peak inductor current with the following
equation:

Note that the peak inductor current is internally limited
to 2A.

Diode Selection

The MAX618’s high switching frequency demands a
high-speed rectifier. Schottky diodes are preferred for
most applications because of their fast recovery time
and low forward voltage. Make sure that the diode’s
peak current rating exceeds the 2A peak switch cur­rent, and that its breakdown voltage exceeds the out­put voltage.

()

MAX618

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

_______________________________________________________________________________________ 9

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

Figure 6. Adding On/Off Control to Circuit of Figure 3

MAX618

VL

OPEN-DRAIN

LOGIC

100k

SHDN

ON/OFF

CONTROL

⎛

V

RR

12 1= −

OUT

⎜

V

⎝

FB

⎞
⎟

⎠

MAX618

IN

SYSTEM

LOGIC SUPPLY

SYSTEM LOGIC

ON/OFF

CONTROL

VL

SHDN

V

OUT

L

=

5

⋅710

I I

LX(PEAK) OUT

=+

V

OUT

V

IN

µ

2s

⎛

⎞

V

IN

⎜

⎟

⎜

L

⎜

⎠

⎝

VV

OUT IN

V

OUT

⎛
⎜

⎝

⎞

−
⎟

⎟
⎟

⎠

MAX618

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

10 ______________________________________________________________________________________

Maximum Output Current

The MAX618’s 2.2A LX current limit determines the
output power that can be supplied for most applica­tions. In some cases, particularly when the input volt­age is low, output power is sometimes restricted by
package dissipation limits. The MAX618 is protected
by a thermal shutdown circuit that turns off the switch
when the die temperature exceeds +150°C. When the
device cools by 10°C, the switch is enabled again.
Table 3 details output current with a variety of input and
output voltages. Each listing in Table 3 is either the limit
set by an LX current limit or by package dissipation at
+85°C ambient, whichever is lower. The values in Table
3 assume a 40mΩ inductor resistance.

Capacitor Selection

Input Capacitors

The input bypass capacitor, C

IND

, reduces the input
ripple created by the boost configuration. High-imped­ance sources require high C

IND

values. However, 68µF
is generally adequate for input currents up to 2A. Low
ESR capacitors are recommended because they will
decrease the ripple created on the input and improve
efficiency. Capacitors with ESR below 0.3Ω are gener­ally appropriate.

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

Output Capacitor

Use Table 4 to find the minimum output capacitance
necessary to ensure stable operation. In addition,
choose an output capacitor with low ESR to reduce the
output ripple. The dominant component of output ripple
is the product of the peak-to-peak inductor ripple cur­rent and the ESR of the output capacitor. ESR below
50mΩ generates acceptable levels of output ripple for
most applications.

Integrator Capacitor

The compensation capacitor (C

COMP

) sets the domi­nant pole in the MAX618’s transfer function. The proper
compensation capacitance depends upon output
capacitance. Table 5 shows the capacitance value

needed for the output capacitances specified in Table

4. However, if a different output capacitor is used (e.g.,
a standard value), then recalculate the value of capaci­tance needed for the integrator capacitor with the fol­lowing formula:

Pole Compensation Capacitor

The pole capacitor (CP) cancels the unwanted zero
introduced by C

OUT

’s ESR, and thereby ensures stabil­ity in PWM operation. The exact value of the pole
capacitor is not critical, but it should be near the value
calculated by the following equation:

where R

ESR

is C

OUT

’s ESR.

Layout Considerations

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

It is important to connect the GND pin, the input
bypass capacitor ground lead, and the output filter
capacitor ground lead to a single point to minimize
ground noise and improve regulation. Also, minimize
lead lengths to reduce stray capacitance, trace resis­tance, and radiated noise, with preference given to the
feedback circuit, the ground circuit, and LX. Place the
feedback resistors as close to the FB pin as possible.
Place a 1µF input bypass capacitor as close as possi­ble to IN and GND.

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

C

COMP

C Table C

COMP OUT

=

C Table

OUT

⋅( )

5

( )

4

R C (R2 R2)

⋅

P

=

ESR OUT

R1 R2

C

+

⋅

MAX618

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

______________________________________________________________________________________ 11

Table 3. Typical Output Current vs. Input and Output Voltages

OUT

V

45678910111213141516171819202122232425262728

IN

V

3 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

4 0.96 0.76 0.64 0.56 0.49 0.43 0.38 0.34 0.31 0.28 0.26 0.24 0.22 0.21 0.19 0.18 0.17 0.16 0.16 0.15 0.14 0.14 0.13 0.12

5 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

6 1.18 0.99 0.85 0.76 0.68 0.63 0.58 0.54 0.50 0.46 0.42 0.39 0.37 0.34 0.32 0.31 0.29 0.28 0.26 0.25 0.24 0.23

7 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

8 1.32 1.13 1.00 0.90 0.82 0.76 0.71 0.66 0.62 0.59 0.56 0.53 0.50 0.47 0.44 0.42 0.40 0.38 0.36 0.35

9 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

10 1.41 1.24 1.11 1.01 0.93 0.86 0.81 0.76 0.72 0.68 0.65 0.62 0.59 0.57 0.55 0.52 0.50 0.47

11 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

12 1.47 1.31 1.19 1.10 1.02 0.95 0.89 0.84 0.80 0.76 0.73 0.70 0.67 0.64 0.62 0.60

13 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

14 1.52 1.37 1.26 1.16 1.09 1.02 0.96 0.91 0.87 0.83 0.79 0.76 0.73 0.71

15 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

16 1.55 1.42 1.31 1.22 1.14 1.08 1.02 0.97 0.93 0.89 0.85 0.82

17 1.57 1.44 1.33 1.25 1.17 1.11 1.05 1.00 0.95 0.91 0.88

18 1.58 1.46 1.36 1.27 1.20 1.13 1.07 1.02 0.98 0.94

19 1.59 1.47 1.37 1.29 1.22 1.15 1.10 1.05 1.00

20 1.60 1.49 1.39 1.31 1.24 1.18 1.12 1.07

21 1.61 1.50 1.41 1.33 1.26 1.20 1.14

22 1.62 1.51 1.42 1.35 1.28 1.22

23 1.63 1.53 1.44 1.36 1.29

24 1.64 1.54 1.45 1.38

25 1.64 1.55 1.46

26 1.65 1.56

27 1.66

MAX618

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

12 ______________________________________________________________________________________

Table 4. Minimum C

OUT

for Stability (µF)

V

OUT

IN

V

45678910111213141516171819202122232425262728

3 173 128 100 80 65 54 46 40 35 31 28 25 23 21 19 18 17 15 15 14 13 12 12 11 10

4 151 118 96 80 68 59 51 45 39 35 32 29 27 24 23 21 20 18 17 16 15 15 14 13

5 132 107 90 77 67 59 52 46 41 37 34 31 29 26 25 23 21 20 19 18 17 16 15

6 117 97 83 72 64 57 51 46 42 38 35 32 30 28 26 24 23 21 20 19 18 17

7 104 89 77 68 61 55 50 45 42 39 35 33 30 28 26 25 23 22 21 20 19

8 9482726458524844413835333129272524222120

9 86766761555046423937343230292725242321

10 79 70 63 57 52 48 44 41 38 36 34 32 30 28 27 25 24 23

11 73 66 59 54 50 46 43 40 37 35 33 31 29 28 26 25 24

12 68 62 56 51 47 44 41 38 36 34 32 30 29 27 26 25

13 64 58 53 49 45 42 39 37 35 33 31 29 28 27 25

14 60 55 50 47 43 40 38 36 34 32 30 29 27 26

15 56 52 48 44 42 39 37 35 33 31 29 28 27

16 53 49 46 43 40 37 35 33 32 30 29 27

17 50 47 44 41 38 36 34 32 31 29 28

18 48 45 42 39 37 35 33 31 30 28

19 46 43 40 38 36 34 32 30 29

20 43 41 38 36 34 33 31 29

21 42 39 37 35 33 32 30

22 40 38 36 34 32 31

23 38 36 34 33 31

24 37 35 33 32

25 35 34 32

26 34 33

27 33

MAX618

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

______________________________________________________________________________________ 13

Table 5. Minimum C

COMP

for Stability (nF)

OUT

V

45678910111213141516171819202122232425262728

IN

3 40 46 54 64 73 83 94 105 118 130 143 157 172 187 203 219 236 253 271 290 309 329 349 370 391

4 42 45 51 58 66 74 82 91 100 109 119 130 141 152 164 176 188 201 214 228 242 257 272 287

5 43 45 49 54 60 67 75 81 88 96 103 111 120 128 137 147 156 166 176 187 197 209 220

6 44 45 48 52 57 62 68 74 80 86 92 99 105 112 119 127 134 142 150 159 167 176

V

7 45454750545863687479859095101107113119125132139146

8 464547495256606468737883889398103108113119124

9 4646474851545761646873778286919599104109

10 47 46 46 48 50 52 55 58 61 65 69 72 77 81 85 89 93 97

11 47 46 46 48 49 51 54 56 59 62 65 69 72 76 80 84 88

12 48 47 47 47 49 50 52 55 57 60 63 66 69 72 75 79

13 48 47 47 47 48 50 52 54 56 58 61 63 66 69 72

14 49 47 47 47 48 49 51 53 55 57 59 61 64 66

15 49 47 47 47 48 49 50 52 53 55 57 59 62

16 49 48 47 47 48 49 50 51 53 54 56 58

17 49 48 47 47 48 48 49 51 52 53 55

18 50 48 47 47 48 48 49 50 51 53

19 50 48 47 47 48 48 49 50 51

20 50 48 48 47 48 48 49 49

21 50 49 48 47 48 48 48

22 50 49 48 48 48 48

23 50 49 48 48 48

24 51 49 48 48

25 51 49 48

26 51 49

27 51

MAX618

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

14 ______________________________________________________________________________________

Package Information

TRANSISTOR COUNT: 1794

___________________Chip Information

QSOP.EPS