_______________ 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 pro-

prietary 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 oper-

ation at light loads to provide high efficiency over a

wide range of load conditions. The MAX618 control

scheme actively monitors the output current and auto-

matically 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, continu-

ous-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 slope-

compensation signal in one circuit block. When the

switch turns off, energy transfers from the inductor to

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

_______________________________________________________________________________________________________

5

Pin Description

PGND

ECB1Q503L

LX

FB

1

µF

4.7

µF

R1

R2

IN

L

SHDN

GND

COMP

UP TO 28V

3V TO 28V

MAX618

R1

R2

8V

402k

Ω

93.1k

Ω

150

µF12µH

150

µF

220pF

0.082

µF

12V

715k

Ω

100k

Ω

100

µF15µH

100

µH

56pF

0.1

µF

28V

574k

Ω

32.4k

Ω

86

µF39µH33µF

47pF

0.47

µF

Figure 1. Single-Supply Operation

FB

7

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.

IN

10

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

VL

11

Power Ground, source of internal N-channel switch

PGND

13, 14, 15

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

COMP

6

Shutdown Input. A logic low puts the MAX618 in shutdown mode and reduces supply current to 3µA.

SHDN

5

PIN

Drain of internal N-channel switch. Connect the inductor between IN and LX.

LX

2, 3, 4

Ground

GND

1, 8, 9,

12, 16

FUNCTION

NAME