Rainbow Electronics MAX1635 User Manual

Page 17

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.

When using adjustable mode, it is a good idea 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.

Secondary Feedback Regulation Loop

(SECFB or V

)

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
input-output differential voltage. If V

or SECFB falls

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-
ondary capacitor and bringing V

or SECFB back into

regulation. The secondary feedback loop does not
improve secondary output accuracy in normal flyback
mode, where the main (primary) output is heavily
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.

Devices with a 12OUT linear regulator have a V

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,
set the feedback resistor ratio to produce 13.5V.
Otherwise, the SECFB one-shot might be triggered
unintentionally, unnecessarily increasing supply current
and output noise.

12V Linear Regulator Output

(MAX1630/MAX1632/MAX1633/MAX1635)

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-

MAX1631
MAX1634

POSITIVE
SECONDARY
OUTPUT

MAIN
OUTPUT

DH_

V+

SECFB

2.5V REF

1-SHOT

TRIG

DL_

WHERE V

REF

(NOMINAL) = 2.5V

TRIP

= V

REF

(

1 + –––

)

R1
R2

MAX1630–MAX1635

Multi-Output, Low-Noise Power-Supply

Controllers for Notebook Computers

______________________________________________________________________________________

Figure 5. Adjusting the Secondary Output Voltage with SECFB

MAX1630
MAX1632
MAX1633
MAX1635

OUTPUT

+12V OUTPUT
200mA

MAIN
OUTPUT

2N3906

0.1

2.2

Ω

V+

12OUT

DH_

DL_

Figure 6. Increased 12V Linear Regulator Output Current