High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

______________________________________________________________________________________

11

Setting the Low-Battery Detector Voltage

Select R1 between 10k

Ω and 1MΩ.

Connect a pull-up resistor (e.g., 100k

Ω) between LBO

__________Applications Information

Layout Considerations

Due to high current levels and fast switching

waveforms, which radiate noise, proper MAX747 PC

board layout is essential. Protect sensitive analog

grounds by using a star ground configuration. Use an

adequate ground plane and minimize ground noise by

connecting GND, the anode of the steering Schottky

diode, the input bypass capacitor ground lead, and the

output filter capacitor ground lead to a single point

(star ground configuration). Also, minimize lead lengths

to minimize stray capacitance, trace resistance, and

radiated noise. Place bypass capacitor C3 as close as

possible to V+ and GND.

AV+ and CS are the inputs to the differential-input

current-sense amplifier. Use a Kelvin connection

across the sense resistor as shown in Figure 6. Note

that even though AV+ also functions as the supply

voltage for sensitive analog circuitry, a separate AV+

bypass capacitor should not be used. By not using a

capacitor, any noise appearing at the CS input will also

appear at the AV+ input and will appear as a common-

mode signal to the current-sense amplifier. A separate

AV+ capacitor causes the noise to appear only on one

input, and this differential noise will be amplified,

adversely affecting circuit operation.

Similarly, CC (or FB in adjustable-output operation) is a

sensitive input that should not be shorted to any node.

Avoid shorting CC when probing the circuit, as this

may damage the device.

Switching Waveforms

A region exists between CCM and DCM where the

inductor current operates in both modes, as shown in

the Idle-Mode Moderate current EXT waveform in the

Typical Operating Characteristics. As the output

voltage varies, it is fed back into CC and the duty cycle

is adjusted to compensate for this change. The switch

threshold voltage. Once the switch is off, the voltage at

EXT is pulled to V+ and the P-FET drain voltage is a

Schottky diode drop below GND. However, in this “in-

between” mode (due to the changing duty cycle

inherent with DCM), when the device is at maximum

always pulled to V+ because the switch sometimes

turns on again after a minimum off-time before EXT can

be pulled to V+. The result is short spikes that appear

on the EXT waveform in the

Typical Operating

Characteristics.

AC Stability with Low

Input/Output Differentials

At low input/output differentials, the inductor current

cannot slew quickly to respond to load changes, so the

output filter capacitor must hold up the voltage as the

load transient is applied. In Figure 1a’s circuit, for

V+ = 6.5V, increase the output filter capacitor to 700µF

(Sprague 595D low-ESR capacitors) to obtain a

transient response less than 250mV with a load step

increase the output filter capacitor to approximately

1000µF. As V+ increases, the device will no longer be

operating near full duty cycle with light loads, allowing

it to adjust to full duty cycle when the load transient is

applied and, in turn, allowing smaller output filter

capacitors to be used.

Dual-Mode Operation

The MAX747 is designed in either fixed-output mode

(5V-output, FB = GND) or in adjustable mode (FB = 2V)

using a resistor divider. It is not designed to be

switched from one mode to another when powered up;

however, in adjustment mode, switching between two

different resistor dividers is acceptable.

R2

R1

(V

V

V

TRIP

REF

REF

=

−













)

AV+

MAX747

CS

RSENSE

P

EXT

L1

KELVIN SENSE

CONNECTION

V+

Figure 6. Kelvin Connection for Current-Sense Amplifier