Mu lt i-Ou t p u t , Lo w -No is e P o w e r-S u p p ly
Co n t ro lle rs fo r No t e b o o k Co m p u t e rs
0–MAX1635
Remote output-voltage sensing, while not possible in
fixed-output mode due to the combined nature of the
voltage-sense and current-sense inputs (CSL3 and
CSL5), is easy to do in adjustable mode by using the top
of the external resistor divider as the remote sense point.
R2
SECFB
1-SHOT
TRIG
R1
Whe n us ing a d jus ta b le mod e , it is a g ood id e a to
always set the “3.3V output” to a lower voltage than the
“5V output.” The 3.3V output must always be less than
VL, so that the voltage on CSH3 and CSL3 is within the
common-mode range of the current-sense inputs. While
VL is nominally 5V, it can be as low as 4.7V when lin-
early regulating, and as low as 4.2V when automatically
bootstrapped to CSH5.
POSITIVE
SECONDARY
OUTPUT
V+
2.5V REF
DH_
MAIN
OUTPUT
MAX1631
MAX1634
S e c o n d a ry Fe e d b a c k Re g u la t io n Lo o p
DL_
(S ECFB o r V
)
DD
A flyback-winding control loop regulates a secondary
winding output, improving cross-regulation when the
primary output is lightly loaded or when there is a low
R1
+V = V (1 + –––)
WHERE V (NOMINAL) = 2.5V
TRIP
REF
REF
R2
input-output differential voltage. If V
or SECFB falls
DD
Figure 5. Adjusting the Secondary Output Voltage with SECFB
below its regulation threshold, the low-side switch is
turned on for an extra 1µs. This reverses the inductor
(primary) current, pulling current from the output filter
capacitor and causing the flyback transformer to oper-
ate in forward mode. The low impedance presented by
the transformer secondary in forward mode dumps cur-
rent into the secondary output, charging up the sec-
+12V OUTPUT
200mA
12OUT
0.1µF
10µF
V
DD
2N3906
ondary capacitor and bringing V or SECFB back into
DD
0.1µF
10Ω
regulation. The secondary feedback loop does not
improve secondary output accuracy in normal flyback
mod e , whe re the ma in (p rima ry) outp ut is he a vily
loaded. In this condition, secondary output accuracy is
determined by the secondary rectifier drop, transformer
turns ratio, and accuracy of the main output voltage. A
linear post-regulator may still be needed to meet strict
output-accuracy specifications.
0.1µF
MAX1630
MAX1632
MAX1633
MAX1635
V+
V
OUTPUT
DD
2.2µF
DH_
DL_
Devices with a 12OUT linear regulator have a V
pin
DD
MAIN
OUTPUT
that regulates at a fixed 13.5V, set by an internal resis-
tor divider. The MAX1631/MAX1634 have an adjustable
secondary output voltage set by an external resistor
divider on SECFB (Figure 5). Ordinarily, the secondary
regulation point is set 5% to 10% below the voltage nor-
mally produced by the flyback effect. For example, if
the output voltage as determined by turns ratio is 15V,
s e t the fe e d b a c k re s is tor ra tio to p rod uc e 13.5V.
Otherwise, the SECFB one-shot might be triggered
unintentionally, unnecessarily increasing supply current
and output noise.
Figure 6. Increased 12V Linear Regulator Output Current
1 2 V Lin e a r Re g u la t o r Ou t p u t
(MAX1 6 3 0 /MAX1 6 3 2 /MAX1 6 3 3 /MAX1 6 3 5 )
The MAX1630/MAX1632/MAX1633/MAX1635 include a
12V linear regulator output capable of delivering 120mA
of output current. Typically, greater current is available
at the expense of output accuracy. If an accurate output
of more than 120mA is needed, an external pass tran-
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
