Rainbow Electronics MAX782 - Manual (Page 17)

MAX782
Triple-Output Power-Supply
Controller for Notebook Computers
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MOSFETs with low gate-threshold voltage specifica-
tions (i.e., maximum V
GS(TH)
= 2V rather than 3V) are
preferred, especially for high-current (5A) applications.
Output Filter Capacitors (C6, C7, C14)
The output filter capacitors determine the loop stability
and output ripple voltage. To ensure stability, the mini-
mum capacitance and maximum ESR values are:
V
REF
C F > —————————————
V
OUT
x R
CS
x 2 x
π
x GBWP
and,
V
OUT
x R
CS
ESR
CF
< ——————
V
REF
where: C
F
= output filter capacitance, C6 or C7 (F);
V
REF
= reference voltage, 3.3V;
V
OUT
= output voltage, 3.3V or 5V;
R
CS
= sense resistor ( Ω );
GBWP = gain-bandwidth product, 60kHz;
ESR
CF
= output filter capacitor ESR (
Ω
).
Be sure to select output capacitors that satisfy both the
minimum capacitance and maximum ESR require-
ments. To achieve the low ESR required, it may be
appropriate to use a capacitance value 2 or 3 times
larger than the calculated minimum.
The output ripple in continuous-current mode is:
V
OUT(RPL)
= I
LPP(MAX)
x (ESR
CF
+1/(2 x π x f x C
F
)).
In idle-mode, the ripple has a capacitive and resistive
component:
4 x 10 -4 x L
V
OUT(RPL)
(C) = ——————— x
R
CS
2 x C
F
1 1 (
——— + —————
)
Volts
V
OUT
V
IN
- V
OUT
0.02 x ESR
CF
V
OUT(RPL)
(R) = ——————- Volts
R
CS
The total ripple, V
OUT(RPL)
, can be approximated as fol-
lows:
if V
OUT(RPL)
(R) < 0.5 V
OUT(RPL)
(C),
then V
OUT(RPL)
= V
OUT(RPL)
(C),
otherwise, V
OUT(RPL)
= 0.5 V
OUT(RPL)
(C) +
V
OUT(RPL)
(R).
Diode D2
The voltage rating of D2 should be at least 2 x V
IN
+
5V plus a safety margin. A rating of at least 75V is
necessary for the maximum 30V supply. A Schottky
diode is preferable for lower input voltages, and is
required for input voltages under 7V. Use a high-
speed silicon diode (with a higher breakdown voltage
and lower capacitance) for high input voltages. D2’s
current rating should exceed twice the maximum cur-
rent load on VDD.
Diodes D3 and D4
Use 1N5819s or similar Schottky diodes. D3 and D4
conduct only about 3% of the time, so the 1N5819’s
1A current rating is conservative. The voltage rating
of D3 and D4 must exceed the maximum input supply
voltage from the battery. These diodes must be
Schottky diodes to prevent the lossy MOSFET body
diodes from turning on, and they must be placed
physically close to their associated synchronous recti-
fier MOSFETs.
Soft-Start Capacitors (C8, C9)
A capacitor connected from GND to either SS pin caus-
es that supply to ramp up slowly. The ramp time to full
current limit, t
SS
, is approximately 1ms for every nF of
capacitance on SS_, with a minimum value of 10µs.
Typical capacitor values are in the 10nF to 100nF
range; a 5V rating is sufficient.
MAX782
KELVIN SENSE TRACES
SENSE RESISTOR
MAIN CURRENT PATH
FAT, HIGH-CURRENT TRACES
Figure 5. Kelvin Connections for the Current-Sense Resistors