UC1548

UC2548

UC3548

There are several methods to program IOFF. If accurate

maximum current control is required, IOFF must track out-

put voltage. The method shown in Figure 4 derives a

voltage proportional to VIN

• D (where D = duty cycle). In

a buck converter, output voltage is proportional to VIN

•

D. A resistively loaded diode connection to the bootstrap

winding yields a square wave whose amplitude is propor-

tional to VIN and is duty cycle modulated by the control

circuit. Averaging this waveform with a filter generates a

primary side replica of secondary regulated Vo. A single

pole filter is shown, but in practice a two or three pole filter

provides better transient response. Filtered voltage is con-

verted by ROFF to a current to the IOFF pin to control CI

downslope.

If accurate system maximum current is not a critical re-

quirement, Figure 5 shows the simplest method of

downslope generation: a single resistor (ROFF = 40k)

from IOFF to VREF. The discharge current is then 100

µA.

The disadvantage to this approach is that the synthesizer

continues to generate a down slope when the switch is off

even during short circuit conditions. Actual inductor

downslope is closer to zero during a short circuit. The

penalty is that the average current is understated by an

amount approximately equal to the nominal inductor rip-

ple current. Output short circuit is therefore higher than

the designed maximum output current.

A third method of generating IOFF is to add a second

winding to the output inductor core (Figure 6). When the

power switch is off and inductor current flows in the free

wheeling diode, the voltage across the inductor is equal

to the output voltage plus the diode drop. This voltage is

then transformed by the second winding to the primary

side of the converter. The advantages to this approach

are its inherent accuracy and bandwidth. Winding the sec-

ond coil on the output inductor core while maintaining the

required isolation makes this a more costly solution. In the

example, ROFF = Vo / 100

µA. The 4 • ROFF resistor is

added to compensate the one volt input level of the IOFF

pin. Without this compensation, a minor current foldback

behavior will be observed.

INDUCTOR CURRENT WAVEFORM SYNTHESIZER (cont.)

UDG-95041

UDG-95042

UDG-95043

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