MAX1652EEE datasheet(18/28 Pages) MAXIM

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High-Efficiency, PWM, Step-Down

DC-DC Controllers in 16-Pin QSOP

______________________________________________________________________________________

With high input voltages, the required duty factor is

approximately (VOUT + VQ2)/ VIN, where VQ2 is the volt-

age drop across the synchronous rectifier. The

MAX1652’s minimum duty factor is determined by

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.

Secondary Feedback-Regulation Loop

(SECFB Pin)

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-

shot is triggered that extends the low-side switch’s

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.

The secondary output voltage-regulation point is deter-

mined by an external resistor-divider at SECFB. For neg-

ative output voltages, the SECFB comparator is

referenced to GND (MAX1654); for positive output volt-

ages, SECFB regulates at the 2.50V reference

(MAX1652). As a result, output resistor-divider connec-

tions and design equations for the two device types dif-

fer slightly (Figure 7). Ordinarily, the secondary

regulation point is set 5% to 10% below the voltage nor-

mally produced by the flyback effect. For example, if the

MAX1654

NEGATIVE

SECONDARY

OUTPUT

MAIN

OUTPUT

V+

SECFB

1-SHOT

TRIG

0.33

µF

REF

MAX1652

POSITIVE

SECONDARY

OUTPUT

MAIN

OUTPUT

V+

SECFB

2.5V REF

1-SHOT

TRIG

+VTRIP

WHERE VREF (NOMINAL) = 2.5V

= VREF (1 + –––)

-VTRIP

R3 = 100k

Ω (RECOMMENDED)

= -VREF (–––)

Figure 7. Secondary-Output Feedback Dividers

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