5
UCC2941-3/-5/-ADJ
UCC3941-3/-5/-ADJ
Figure 2. Inductor current and output ripple waveforms.
APPLICATION INFORMATION (cont.)
UDG-96117
At time
t1, the 3.3V output drops below its lower thresh-
old, and the inductor is charged with an on time deter-
mined by:
T
s
VIN
ON =
12
µ
For a 1.25V input, and a 22
µH inductor, the resulting
peak current is approximately 500mA. At time
t2, the in-
ductor begins to discharge with a minimum off time of
1.7
µs. Under lightly loaded conditions, the amount of en-
ergy delivered in this single pulse would satisfy the volt-
age control loop, and the converter would not command
any more energy pulses until the output again drops be-
low the lower voltage threshold.
At time
t3, the VGD supply has dropped below its lower
threshold, but the output voltage is still above its thresh-
old point. This results in an energy pulse to the gate drive
supply at
t4. However, while the gate drive is being serv-
iced, 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 mini-
mum 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 0.5A at
the end of the next charge on time. The result is a
ratcheting of inductor current until either the output volt-
age 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, un-
less VGD drops below 7.5V.
Between
t7 and t8, the converter reaches its peak current
limit which is determined by RPL and VIN. Once the limit
is reached, the converter operates in continuous mode
with approximately 200mA of ripple current. At time
t8,
the output voltage is satisfied, and the converter can ser-
vice VGD, which occurs at
t9.