High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

8

_______________________________________________________________________________________

2

3

4

5

11

LBI

SS

REF

SHDN

AGND

V+

CP

HIGH

AV+

CS

EXT

CC

OUT

FB

LB0

GND

C6

1

µF

D2

1N914

C8

0.1

µF

14

13

8

10

12

7

9

6

1

R2

R1

C7

2nF

C2

100

µF

C3

0.1

µF

C5

0.1

µF

D3

1N914

C9

1

µF

C10

0.1

µF

D5

1N914

D4

1N5817

C11

1

µF

R4

20k (1%)

R5

13k (1%)

D1

NSQ03A03

L1

*

22

µH

C4

0.1

µF

N

40m

Ω

R3

100k

Q1

Si9410DY

D6

1N914

3.3V

AT 3A

C3

660

µF

15

16

MAX746

VIN

4.5V TO 6V

with light loads, the PWM comparator trips quickly and the

idle-mode comparator sets the EXT on-time.

Traditional PWM converters continue to switch on every

cycle, even when the inductor current is discontinuous

due to smaller loads, decreasing light-load efficiency.

In contrast, the MAX746’s idle-mode comparator increas-

es the switch on-time, allowing more energy to be trans-

ferred per cycle. Since fewer cycles are required, the

switching frequency is reduced, resulting in minimal

switching losses and increased efficiency.

The light-load output noise spectrum widens due to the

variable switching frequency in idle-mode, but output

ripple remains low. Using the Typical Operating Circuit,

with a 9V input and a 125mA load current, output ripple

is less than 40mV.

Charge Pump

The MAX746 contains all the control circuitry required

to provide a regulated charge-pump voltage 5V

above V+ for high-side driving N-channel logic FETs.

The charge pump operates with a nominal 50kHz fre-

quency. When the voltage at HIGH exceeds AV+ by

5V, the charge-pump oscillator is inhibited (Figure 2).

When the voltage at HIGH is less than 4.3V below V+,

undervoltage lockout occurs. Use the voltage tripler

(Figure 3b) when V+

≤ 6V; otherwise, use the voltage

doubler (Figure 3a).

Soft-Start and Current Limiting

The MAX746 draws its highest current at power-up. If

the power source to the MAX746 cannot provide this

initial elevated current, the circuit may not function cor-

rectly. For example, after prolonged use the increased

series resistance of a battery may prevent it from pro-

viding adequate initial surge currents when the

MAX746 is brought out of shutdown. Using soft-start

(SS) minimizes the possibility of overloading the incom-

ing supply at power-up by gradually increasing the

peak current limit. Connect an external capacitor from

SS to AGND to reduce the initial peak currents drawn

from the supply.

Figure 1b. 3.3V Standard Application Circuit (9.9W)