With decreasing loads, as the inductor current becomes

discontinuous, traditional PWM converters continue to

switch at a fixed frequency, decreasing light-load efficiency.

However, the MAX747’s idle-mode comparator increases

the peak inductor current, allowing more energy to be

transferred per cycle. Since fewer cycles are required, the

switching frequency is reduced. This keeps the external P-

FET off for longer periods, minimizing switching losses and

increasing efficiency.

The light-load output noise spectrum widens due to 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.

Soft-Start and Current Limiting

power source to the MAX747 cannot provide this initial

elevated current, the circuit may not function correctly. For

example, after prolonged use, a battery’s increased series

resistance may prevent it from providing adequate initial

surge currents when the MAX747 is brought out of

shutdown. Using Soft-Start (SS) minimizes the possibility of

overloading the incoming supply at power-up by gradually

increasing the peak current limit. Connect an external

capacitor from SS to ground to reduce the initial peak

currents drawn from the supply.

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

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

3.8V. The current-limit comparator inhibits EXT switching

until the SS voltage reaches 1.8V. The maximum current

limit is set by:

Figure 3 shows how the peak current limit increases as

Shutdown Mode

When SHDN is high, the MAX747 enters shutdown

mode. In this mode, the internal biasing circuitry

supply current drops to 0.6µA (20µA max). This

excludes external component leakage, which may add

several microamps to the shutdown supply current for

the entire circuit. SHDN is a TTL/CMOS logic-level

input. Connect SHDN to GND for normal operation.

Low-Battery Detector

The MAX747 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

the desired level. In this circuit, LBO goes low when

V+

≤ V

__________________Design Procedure

Setting the Output Voltage

The MAX747’s output voltage can be set to 5V by

grounding FB, or adjusted from 2V to 14V using

external resistors R4 and R5, configured as shown in

Figure 5. Select feedback resistor R4 from the 10k

Ω to

1M

Ω range. R5 is given by:

Selecting RSENSE

Once all component values have been determined, the

actual peak current is given by:

125mV/3.3A = 38m

Ω.

The sense resistor should have a power rating greater

The power dissipated by the resistor (assuming an 80%

R

V

I

125mV

I

SENSE

LIMIT (MIN)

PK

PK

==

I

I

V

(2L) (f

)

1

V

V

PK

LOAD

OUT

OSC

OUT

IN

=+













−









R5

(R4)

V

2V

1

OUT

=−













I

V

R

150mV (typ)

R

PK

LIMIT

SENSE

SENSE

==

High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

8

_______________________________________________________________________________________

0

SOFT-START VOLTAGE (V)

4

2

1

13

3

2

0

Figure 3. Peak Current Limit vs. Soft-Start Voltage