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

8

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GND

LX

L

PGND

R2

R1

VL

COMP

IN

SHDN

UP TO 28V

UP TO 28V

IN

OUT

MAX618

2.7V TO 5.5V

4.7

µF

1

µF

FB

LX

L

PGND

GND

R1

VL

COMP

IN

SHDN

UP TO 28V

UP TO 28V

OUT

MAX618

IN

3V TO 28V

4.7

µF

1

µF

R2

FB

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 power-

in Figure 3.

Operating Configurations

The MAX618 can be connected in one of three configura-

tions 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 pro-

viding 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

RANGE

(up to 28V)

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

(up to 28V)

(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

• 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 volt-

age supplied by sepa-

rate sources.

IN and VL connect

together. Inductor volt-

age supplied by a

separate source.

2.7V to 5.5V

3V to 28V