R2 and C4 form a lowpass filter to remove switching

fairly low ESR (<5

Ω). Switching noise can interfere with

proper output voltage regulation, resulting in an exces-

sive output voltage decrease (>100mV) at full load.

Overheating during soldering can damage the surface-

mount capacitors specified for C4, causing the regula-

tion problems described above. Take care to heat the

capacitor for as short a time as possible, especially if it

is soldered by hand.

Rectifier, D2

Use a 1N5817 or similar Schottky diode for applications

up to 3A, or a 1N5820 for up to 10A. Surface-mount

equivalents are available from N.I.E.C. with part num-

bers EC10QS02 and NSQ03A02, or from Motorola with

part numbers MBRS120T3 and MBRS320T3. D2 must

be a Schottky diode to prevent the lossy MOSFET body

diode from turning on.

Soft-Start

A capacitor connected from GND to SS causes the

supply’s current-limit level to ramp up slowly. The ramp

time to full current limit is approximately 1ms for every

nF of capacitance on SS, with a minimum value of

10µs. Typical values for the soft-start capacitor are in

the 10nF to 100nF range; a 5V rating is sufficient.

The time required for the output voltage to ramp up to

its rated value depends upon the output load, and is

not necessarily the same as the time it takes for the cur-

rent limit to reach full capacity.

Duty Cycle

The duty cycle for the high-side MOSFET (N1) in con-

tinuous-conduction mode is:

___________________

where:

It is apparent that, in continuous-conduction mode, N1

will conduct for about twice the time as N2. Under short-

circuit conditions, however, N2 can conduct as much

90% of the time. If there is a significant chance of short

circuiting the output, select N2 to handle the resulting

duty cycle (see

Short-Circuit Duration section).

MOSFET Switches, N1 and N2

The two N-channel MOSFETs must be “logic-level”

FETs; that is, they must be fully on (have low

high-current applications, FETs with low gate-

threshold voltage specifications (i.e., maximum

tion, they should have low total gate charge (<70nC)

to minimize switching losses.

For output currents in excess of the five standard appli-

cation circuits, placing MOSFETs with very low gate

charge in parallel increases output current and lowers

resistive losses. N2 does not normally require the same

current capacity as N1 because it conducts only about

33% of the time, while N1 conducts about 66% of the

time.

Short-Circuit Duration

At their highest rated temperatures (+70°C or +85°C),

each of the five standard application circuits will with-

stand a short circuit of several seconds duration. In

most cases, the MAX767 will be used in applications

where long-term short circuiting of the output is unlikely.

If it is desirable for the circuit to withstand a continuous

short circuit, the MOSFETs must be able to dissipate

the required power. This depends on physical factors

such as the mounting of the transistor, any heat-

sinking used, and ventilation provided, as well as the

actual current the transistor must deliver. The short-

circuit current is approximately 100mV / R1, but may

vary by ±20%.

Cautious design requires that the transistors

withstand the maximum possible current, which is

current scaled by their maximum duty factors. The

maximum duty factor for N1 occurs under high-

load (but not short-circuit) conditions, and is approxi-

imum duty factor for N2 occurs during short-circuit

conditions and is:

1 - —————————————

which can exceed 0.9. The total power dissipated in

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

______________________________________________________________________________________

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