Datasheet

( )
TMIN
TMIN 5.92 R ns= ´
5 125
R
TMIN
- Resistor Setting - kW
900
65
TMIN - Minimum Time - ns
MINIMUM TIME
vs
RESISTOR SETTING
800
200
0
600
400
100
500
15
25
35 55 75 85 95 115
45 105
300
700
( )
4
SW(osc)
DMIN TMIN F 10 %
-
= ´ ´
UCC28950
SLUSA16B –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
Minimum Pulse (TMIN)
The resistor R
TMIN
from TMIN pin to GND sets fixed minimum pulse TMIN applied to the output rectifier enabling
ZVS of the primary switches at light load. If the output PWM pulse demanded by the feedback loop is shorter
than TMIN, then controller proceeds to the burst mode of operation where even number of TMIN pulses are
followed by the off time dictated by the feedback loop. The proper selection of TMIN duration is dictated by the
time it takes to raise the sufficient magnetizing current in the power transformer to maintain ZVS. The minimum
pulse TMIN is defined by the following Equation 8.
(8)
In this equation R
TMIN
is in kΩ and TMIN is in ns.
NOTE
The minimum allowed resistor on TMIN, R
TMIN
is 13 kΩ.
The related plot is shown in Figure 10
Figure 10. Minimum Time TMIN Over Setting Resistor R
TMIN
The value of minimum duty cycle DMIN is determined by Equation 9.
(9)
Here, F
SW(osc)
is oscillator frequency in kHz, TMIN is the minimum pulse in ns and DMIN is in percents.
Burst Mode
If the converter is commanding a duty cycle lower than TMIN, then the controller will go into Burst Mode. The
controller will always deliver even number of Power cycles to Power transformer. The controller always stops its
bursts with OUTB and OUTC power delivery cycle. If the controller is still demanding a duty cycle less than
TMIN, then the controller goes into shut down mode. Then it waits until the converter is demanding a duty cycle
equal or higher than TMIN before the controller puts out TMIN or a PWM duty cycle as dictated by COMP
voltage pin.
18 Submit Documentation Feedback Copyright © 2010–2011, Texas Instruments Incorporated
Product Folder Link(s): UCC28950