NCP1237_11 Datasheet, PDF (22/41 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller for Flyback Converters

NCP1237
Oscillator with Maximum Duty Ratio and Frequency
Jittering
The NCP1237 includes an oscillator that sets the
switching frequency with an accuracy of $7%. The
maximum duty ratio of the DRV pin is 80% (typical), with
an accuracy of $7%.
f_OSC
In order to improve the EMI signature, the switching
frequency jitters around its nominal value, with a
triangle−wave shape.
f_OSC + A_jitter
Nominal f_OSC
f_OSC - A_jitter
1 / F_jitter
Figure 48. Frequency Jittering
Time
Clamped Driver
The supply voltage for the NCP1237 can be as high as
28 V, but most of the MOSFETs that will be connected to the
DRV pin cannot tolerate a gate−to−source voltage greater
than 20 V on their gate. The driver pin is therefore clamped
safely below 16 V.
This driver has a typical current capability of ±1.0 A.
VCC
Clamp
DRV signal
DRV
Figure 49. Clamped Driver
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