Rainbow Electronics MAX767 User Manual

Page 13

RC Filter for V

R2 and C4 form a lowpass filter to remove switching
noise from the V

input to the MAX767. C4 must have

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:

100% x ( V

OUT

+ V

)

___________________

- V

where:

OUT

= 3.3V

= 5V

and V

= I

LOAD

x r

DS(ON)

for each MOSFET.

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”
F E T s ; t h a t i s , t h e y m u s t b e f u l l y o n ( h a v e l o w
r

DS(ON)

) with only 4V gate-source drive voltage. For

h i g h - c u r r e n t a p p l i c a t i o n s , F E T s w i t h l o w g a t e -
threshold voltage specifications (i.e., maximum
V

GS(TH)

= 2V rather than 3V) are preferred. In addi-

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
I

= 120mV / R1. N1 and N2 must withstand this

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-
mately V

OUT

/ V

(min) or about 0.7. The max-

imum duty factor for N2 occurs during short-circuit
conditions and is:

x r

DS(ON)N2

1 - —————————————

(max) - I

x r

DS(ON)N1

which can exceed 0.9. The total power dissipated in
both MOSFETs together is I

x r

DS(ON)

MAX767

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

Power-Supply Controller

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