High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

______________________________________________________________________________________

15

Increasing C7 by up to 50% enhances outer-loop

stability by adding stability to the inductor current

waveform. But increasing C7 too much causes

FB’s response time to decrease (due to the larger

RC time constant caused by the feedback resistors

and the compensation capacitor), which reduces

load-transient stability.

Setting the Low-Battery

Detector Voltage

Select R1 between 10k

Ω and 1MΩ. Determine R2 using

the following equation:

R2 = R1

(e.g., 100k

Using a Second Supply in

Place of the Charge Pump

If a secondary power supply (a minimum of 5V above

the main supply) is available, it can be substituted for

the charge-pump high-side supply. In this case,

bypass HIGH with a 1µF capacitor and leave CP

unconnected. Since this secondary supply voltage

gate-source breakdown voltage of the external N-FET.

Also, the voltage at HIGH must not exceed 20V. If

a secondary supply is used, the shutdown function

cannot be used because HIGH is internally tied to

V+ in shutdown mode. In this case, SHDN must be

tied low. With the main supply off and HIGH at 12V,

HIGH will typically sink 130µA.

Layout Considerations

Because high current levels and fast switching wave-

forms radiate noise, proper PC board layout is essen-

tial. Use a 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

reduce stray capacitance, trace resistance, and radiat-

ed noise. Place bypass capacitor C3 as close to V+

and GND as possible.

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 8.

Although AV+ also functions as the supply voltage

for sensitive analog circuitry, a separate AV+ bypass

capacitor should not be used. By not using a capaci-

tor, any noise at the CS input will also appear at the

AV+ input, and will be interpreted by the current-

sense amplifier as a common-mode signal . A sepa-

rate AV+ capacitor causes the noise to appear on

only one input, and this differential noise will be

amplified, adversely affecting circuit operation.

Additional Notes

When probing the MAX746 circuit, avoid shorting

V+ to GND (the two pins are adjacent) as this may

cause the IC to malfunction because of large ground

currents. Because of its fast switching and high drive-

capability requirements, EXT is a low-impedance point

that is not short-circuit protected. Therefore, do not

short EXT to any node (including AGND and V+, which

are adjacent to EXT).

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.

The MAX746 may continue to operate with AV+ discon-

nected, but erratic switching waveforms will appear at EXT.

Switching Waveforms

There is a region 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 out-

put voltage varies, it is fed back into CC and the

duty cycle adjusts to compensate for this change.

AV+

CS

RSENSE

N

EXT

L1

KELVIN SENSE

CONNECTION

V+

MAX746

Figure 8. Kelvin Connection for Current-Sense Amplifier