UCC2941-3, UCC2941-5, UCC2941-ADJ, UCC3941-3, UCC3941-5, UCC3941-ADJ

1V SYNCHRONOUS BOOST CONVERTER

SLUS242B – JANUARY 1999 – REVISED JUNE 2001

8

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APPLICATION INFORMATION

UDG–96117

Figure 2. Inductor Current and Output Ripple Waveforms

At time t1, the 3.3-V output drops below its lower threshold, and the inductor is charged with an on time

determined by:

t

ON +

12

ms

VIN

For a 1.25-V input, and a 22-

µH inductor, the resulting peak current is approximately 500 mA. At time t2, the

inductor begins to discharge with a minimum off time of 1.7

µs. Under lightly loaded conditions, the amount of

energy delivered in this single pulse satisfies the voltage-control loop, and the converter does not command

any more energy pulses until the output again drops below the lower voltage threshold.

At time t3, the VGD supply has dropped below its lower threshold, but the output voltage is still above its

threshold point. This results in an energy pulse to the gate drive supply at t4. However, while the gate drive is

being serviced, the output voltage has dropped below its lower threshold, so the state machine commands an

energy pulse to the output as soon as the gate drive pulse is completed.

Time t6, represents a transition between light and heavy load. A single energy pulse is not sufficient to force

the output voltage above its upper threshold before the minimum off-time has expired, and a second charge

cycle is commanded. Since the inductor current does not reach zero in this case, the peak current is greater

than 0.5 A at the end of the next charge on time. This results in a ratcheting of the inductor current until either

the output voltage is satisfied, or the converter reaches its programmed current limit. At time t7, the gate drive

voltage has dropped below its threshold but the converter continues to service the output because it has highest

priority, unless VGD drops below 7.5 V.

limit is reached, the converter operates in continuous mode with approximately 200 mA of ripple current. At time

t8, the output voltage is satisfied, and the converter can service VGD, which occurs at t9.

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