LT1737CS datasheet(25/28 Pages) LINER

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LT1737

1737f

therefore requires a physically larger transformer. The larg-

est size VERSA-PAC is used, a VP5-0083. Three windings

are paralleled for both the primary and secondary.

Overall power supply efficiency and output regulation ver-

sus load current at the nominal VIN = 5V may be seen in

Figures 16 and 17.

TYPICAL APPLICATIO S

NONISOLATED APPLICATION

While the LT1737 was designed to serve isolated flyback

applications, it is useful to note that it is also capable of

supporting nonisolated applications. These are performed

by providing a continuous pseudo-DC feedback signal to

the FB pin. (The part behaves as if the flyback waveform

is infinitely long.) Figure 18 demonstrates just such a

system.

A SEPIC topology is shown whereby a 8V to 16V input is

converted to a nonisolated 12V output. A conventional

resistive feedback divider, R3/R4 drives the FB pin. (Ca-

pacitor C7 serves to filter out high frequency ripple in the

output voltage.) A combination of an R/C network (R11/

C5) in parallel with a single capacitor (C9) on the VC node

provides the required loop compensation. The load com-

pensation function is unwanted, so the ROCMP pin is left

open and the RCMPC pin is grounded. An LT1121 low

dropout regulator is programmed to a nominal 8.25V

output by the R12/R13 resistor divider, and this allows the

LT1737 to drive M1, a logic level MOSFET. Minimum on

time programming resistor R5 is set to 33k to minimize the

required output preload. Minimum enable time has no

direct effect on steady state operation, but programming

resistor R7 has been set to 100k for rapid start-up. Enable

delay resistor is similarly set to 24k.

Overall power supply efficiency versus input voltage and

load current may be seen in Figure 19. Because this

application example utilizes a nonisolated topology, load

regulation is not an issue. It is typically 0.2% (25mV) from

no load to full load.

Other nonisolated switching topologies may be similarly

implemented. For example, Boost and NonIsolated Fly-

back readily suggest themselves. (A Nonisolated Flyback

topology also can be used to generate a

negative output

voltage. In this case, the feedback is a dynamic waveform

derived from the primary side of the transformer, similar

to an isolated LT1737 application.)

0.01

0.1

1737 F16

ILOAD (A)

Figure 16. Efficiency vs ILOAD

ILOAD (A)

5.00

1737 F17

4.75

5.25

Figure 17. Load Regulation

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