MAX782 MAXIM [Maxim Integrated Products], MAX782 Datasheet - Page 17

• Manufacturer Part Number: MAX782
• Description: Triple-Output Power-Supply Controller for Notebook Computers
• Manufacturer: MAXIM [Maxim Integrated Products]

MOSFETs with low gate-threshold voltage specifica-
tions (i.e., maximum V
preferred, especially for high-current (5A) applications.
The output filter capacitors determine the loop stability
and output ripple voltage. To ensure stability, the mini-
mum capacitance and maximum ESR values are:
and,
where:
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.
Figure 5. Kelvin Connections for the Current-Sense Resistors
KELVIN SENSE TRACES
Output Filter Capacitors (C6, C7, C14)
C
ESR
C
V
V
R
GBWP = gain-bandwidth product, 60kHz;
ESR
REF
OUT
F
F
CS
= output filter capacitance, C6 or C7 (F);
> —————————————
CF
CF
= sense resistor (Ω);
= reference voltage, 3.3V;
= output voltage, 3.3V or 5V;
V
MAX782
< ——————
= output filter capacitor ESR (Ω).
OUT
______________________________________________________________________________________
V
OUT
x R
GS(TH)
V
REF
CS
FAT, HIGH-CURRENT TRACES
x R
V
MAIN CURRENT PATH
x 2 x π x GBWP
REF
CS
= 2V rather than 3V) are
Controller for Notebook Computers
SENSE RESISTOR
Triple-Output Power-Supply
The output ripple in continuous-current mode is:
V
In idle-mode, the ripple has a capacitive and resistive
component:
The total ripple, V
lows:
The voltage rating of D2 should be at least 2 x V
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.
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.
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
capacitance on SS_, with a minimum value of 10µs.
Typical capacitor values are in the 10nF to 100nF
range; a 5V rating is sufficient.
OUT(RPL)
otherwise, V
then V
= I
V
(
V
——— + —————
if V
OUT(RPL)
OUT(RPL)
V
LPP(MAX)
1
V
OUT
SS
OUT(RPL)
OUT(RPL)
OUT(RPL)
OUT(RPL)
, is approximately 1ms for every nF of
Soft-Start Capacitors (C8, C9)
OUT(RPL)
(C) = ——————— x
(R) = ——————- Volts
x (ESR
V
IN
(R) < 0.5 V
(R).
= V
= 0.5 V
1
- V
, can be approximated as fol-
0.02 x ESR
OUT(RPL)
4 x 10
CF
OUT
R
CS
+1/(2 x π x f x C
OUT(RPL)
R
2
)
Diodes D3 and D4
CS
-4
OUT(RPL)
x C
Volts
(C),
x L
CF
F
(C) +
(C),
Diode D2
F
)).
IN
17
+