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APPLICATION INFORMATION

OPERATION

UCC29411, UCC29412

UCC29413, UCC39411

UCC39412, UCC39413

SLUS245E – MARCH 2000 – REVISED JULY 2005

A detailed block diagram of the UCC39411 is shown in Figure 1. Unique control circuitry provides high-efficiency

power conversion for both light and heavy loads by transitioning between discontinuous and continuous

conduction based on load conditions. Figure 2 depicts converter waveforms for the application circuit shown in

Figure 3. A single 22µH inductor provides the energy pulses required for a highly efficient 3.3 V converter at up

to 200 mW output power.

At time t1, the 3.3 V output voltage has dropped below its lower threshold, and the inductor is charged with an on

approximately 250 mA. At time t2, the inductor begins to discharge with a minimum off time of approximately 1

ms. 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 drops 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. In some cases, a single pulse supplied to

VGD is insufficient to raise the VGD voltage level enough to satisfy the voltage loop. Under this condition,

multiple pulses are supplied to VGD. Note that when the UCC3941x is servicing VGD only, the IC maintains a

discontinuous mode of operation. After time t4, the 3.3 V output drops below its threshold and requests to be

serviced once the VGD cycle has completed, which occurs at time t5.

Time t6 represents a transition between light load 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 250

mA at the end of the next charge on time. The result is a ratcheting of inductor current until either the output

voltage is satisfied, or the converter reaches its set current limit. At time t7, the gate drive voltage has dropped

below its 7 V threshold but the converter continues to service the output because it has higher priority unless

VGD drops below about 6.3 V.

Between time t7 and t8, the converter reaches its peak current limit.

Once the peak current is reached, the converter operates in continuous mode with approximately 60 mA of

inductor current ripple. At time t8, the 3.3 V output is satisfied and the converter can service the gate drive

voltage, VGD, which occurs at time t9.

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