MAX746 データシートの表示（PDF） - Maxim Integrated

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MAX746

High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller

Maxim Integrated 

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

VIN

4.5V TO 6V

C3

0.1µF

C2

100µF

D2 D3 D5 D6

D4

C11

1N914 1N914 1N914 1N914

1N5817

1µF

R2

15

C8

C9 C10

2 LBI

V+

CP 14 0.1µF 1µF 0.1µF

R1

C5

0.1µF 3 SS

HIGH 13

AV+ 8

C6

1µF 4 REF MAX746 CS 10

RSENSE

40mΩ

Q1

EXT 12

Si9410DY

N L1*

5 SHDN

11 AGND

CC 7

OUT 9

FB 6

C7

2nF

1

LB0

R5

13k (1%)

R4

20k (1%)

D1

NSQ03A03

22µH

R3

100k

GND

16

C4

0.1µF

3.3V

AT 3A

C3

660µF

* SUMIDA CDR125 22µH SURFACE-MOUNT INDUCTOR

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

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.

8 _______________________________________________________________________________________

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