Hig h -Effic ie n c y, P WM, S t e p -Do w n
DC-DC Co n t ro lle rs in 1 6 -P in QS OP
With high input voltages, the required duty factor is
approximately (V + V )/ V , where V is the volt-
on-time beyond the point where the inductor current
crosses zero (in discontinuous mode). This causes the
inductor (primary) current to reverse, which in turn pulls
current out of the output filter capacitor and causes the
flyback transformer to operate in the forward mode. The
low impedance presented by the transformer secondary
in the forward mode dumps current into the secondary
output, charging up the secondary capacitor and bring-
ing SECFB back into regulation. The SECFB feedback
loop does not improve secondary output accuracy in
normal flyback mode, where the main (primary) output is
heavily loaded. In this mode, secondary output accura-
cy is determined (as usual) by the secondary rectifier
drop, turns ratio, and accuracy of the main output volt-
age. Hence, a linear post-regulator may still be needed
in order to meet tight output accuracy specifications.
OUT
Q2
IN
Q2
a g e d rop a c ros s the s ync hronous re c tifie r. The
MAX1652’s minimum d uty fa c tor is d e te rmine d b y
delays through the feedback network, error comparator,
internal logic gate drivers, and the external MOSFETs,
which typically total 400ns. This delay is about 12% of
the switching period at 300kHz and 6% at 150kHz, limit-
ing the typical minimum duty factor to these values.
Even if the circuit can not attain the required duty factor
dictated by the input and output voltages, the output
voltage will remain in regulation. However, there may be
intermittent or continuous half-frequency operation. This
can cause a factor-of-two increase in output voltage rip-
ple and current ripple, which will increase noise and
reduce efficiency. Choose 150kHz operation for high-
input-voltage/low-output-voltage circuits.
The secondary output voltage-regulation point is deter-
mined by an external resistor-divider at SECFB. For neg-
a tive outp ut volta g e s , the SECFB c omp a ra tor is
referenced to GND (MAX1654); for positive output volt-
a g e s , SECFB re g ula te s a t the 2.50V re fe re nc e
(MAX1652). As a result, output resistor-divider connec-
tions and design equations for the two device types dif-
fe r s lig htly (Fig ure 7). Ord ina rily, the s e c ond a ry
regulation point is set 5% to 10% below the voltage nor-
mally produced by the flyback effect. For example, if the
2–MAX165
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
(S ECFB P in )
A flyback winding control loop regulates a secondary
winding output (MAX1652/MAX1654 only), improving
cross-regulation when the primary is lightly loaded
or when there is a low input-output differential voltage.
If SECFB crosses its regulation threshold, a 1µs one-
s hot is triggered that extends the low-side switch’s
0.33µF
REF
R3
R3
SECFB
SECFB
1-SHOT
1-SHOT
TRIG
TRIG
R2
R2
POSITIVE
NEGATIVE
SECONDARY
OUTPUT
V+
V+
2.5V REF
DH
SECONDARY
OUTPUT
DH
DL
MAIN
OUTPUT
MAX1652
MAX1654
MAIN
OUTPUT
DL
R2
+V = V (1 + –––)
R2
-V = -V (–––)
WHERE V (NOMINAL) = 2.5V
R3 = 100kΩ (RECOMMENDED)
TRIP
REF
REF
TRIP
REF
R3
R3
Figure 7. Secondary-Output Feedback Dividers
18 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
