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LT1737

1737f

SELECTING OSCILLATOR CAPACITOR VALUE

The switching frequency of the LT1737 is set by an

external capacitor connected between the OSCAP pin and

ground. Recommended values are between 200pF and

33pF, yielding switching frequencies between 50kHz and

250kHz. Figure 2 shows the nominal relationship between

external capacitance and switching frequency. To mini-

mize stray capacitance and potential noise pickup, this

capacitor should be placed as close as possible to the IC

and the OSCAP node length/area minimized.

APPLICATIO S I FOR ATIO

indicative of actual current level in the transformer pri-

mary, and may cause irregular current mode switching

action, especially at light load.

However, the user must remember that the LT1737 does

not “skip cycles” at light loads. Therefore, minimum on

time will set a limit on minimum delivered power and con-

sequently a minimum load requirement to maintain regu-

lation (see Minimum Load Considerations). Similarly,

minimum on time has a direct effect on short-circuit be-

havior (see Maximum Load/Short-Circuit Considerations).

The user is normally tempted to set the minimum on time

to be short to minimize these load related consequences.

(After all, a smaller minimum on time approaches the ideal

case of zero, or no minimum.) However, a longer time may

be required in certain applications based on MOSFET

switching current spike considerations.

Enable Delay Time

This function provides a programmed delay between

turnoff of the gate drive node and the subsequent enabling

of the feedback amplifier. At high loads, a primary side

voltage spike after MOSFET turnoff may be observed due

to transformer leakage inductance. This spike is not in-

dicative of actual output voltage (see Figure 4B). Delaying

the enabling of the feedback amplifier allows this system

to effectively ignore most or all of the voltage spike and

maintain proper output voltage regulation. The enable

delay time should therefore be set to the maximum ex-

pected duration of the leakage spike. This may have

COSCAP (pF)

100

300

100

200

1737 F02

Figure 2. fOSC vs OSCAP Value

SELECTING TIMING RESISTOR VALUES

There are three internal “one-shot” times that are pro-

grammed by external application resistors: minimum on

time, enable delay time and minimum enable time. These

are all part of the isolated flyback control technique, and

their functions have been previously outlined in the Opera-

tion section. Figure 3 shows nominal observed time ver-

sus external resistor value for these functions.

The following information should help in selecting and/or

optimizing these timing values.

Minimum On Time

This time defines a period whereby the normal switch

current limit is ignored. This feature provides immunity to

the leading edge current spike often seen at the source

node of the external power MOSFET, due to rapid charging

of its gate/source capacitance. This current spike is not

RT (kΩ)

100

500

1000

100

250

1737 F03

Figure 3. “One Shot” Times vs Programming Resistor

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