High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

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9

The steady-state SS pin voltage is typically 3.8V. On

power-up, SS sources 1µA until its voltage reaches

3.8V. The current-limit comparator inhibits EXT switch-

ing until the SS voltage reaches 1.8V. The peak current

limit is set by:

150mV (typ)

sense amplifier inputs. Figure 4 shows how the SS peak

current limit increases as the voltage on SS rises for two

Undervoltage Lockout

Undervoltage lockout inhibits operation of EXT until the

charge pump is capable of generating a voltage greater

than 4.3V above the supply voltage (Figure 2). When

the undervoltage-lockout comparator detects an under-

voltage condition, the switching action at EXT is halted.

Shutdown Mode

When SHDN is high, the MAX746 is shut down. In this

mode, the internal biasing circuitry (including EXT) is

drops to 1.4µA (20µA max). This excludes external

component leakage, which may add several

microamps to the shutdown supply current for the

entire circuit. SHDN is a logic input. Connect SHDN to

GND for normal operation.

Low-Battery Detector

The MAX746 provides a low-battery comparator that

compares the voltage on LBI to the reference voltage.

LBO, an open-drain output, goes low when the LBI volt-

shown in the Input Voltage Monitor Circuit (Figure 5),

this circuit, LBO goes low when V+

impedance in shutdown mode.

2

3

4

6

5

11

LBI

SS

REF

FB

SHDN

AGND

V+

CP

HIGH

AV+

CS

EXT

CC

OUT

LB0

VIN

6V TO 15V

GND

C6

1

µF

D2

1N914

*

C8

0.1

µF

D4

1N5817

D3

1N914

*

D1

NSQ03A03

5V

AT 1.5A

L1

**

82

µH

C1

220

µF

N

C9

4.7

µF

14

13

8

10

12

7

9

1

R2

R1

75m

Ω

R3

100k

C7

1nF

C4

0.1

µF

C2

47

µF

C3

0.1

µF

C5

0.1

µF

Q1

Si9410DY

15

16

MAX746

Figure 1c. 5V Standard Application Circuit (7.5W)