UCC28951
www.ti.com.cn
ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
There is a programmable delay for the turnoff of FET QF after FET QA turnoff (tAFSET) and the turnoff of FET QE
after FET QB turnoff (tBESET). Set these delays to 50% of tABSET to ensure that the appropriate synchronous
rectifier turns off before the AB ZVS transition. If this delay is too large, it causes OUTE and OUTF not to overlap
correctly and creates excess body diode conduction on FETs QE and QF.
tAFSET = tBESET = tABSET ´0.5
(137)
The resistor divider formed by RAEF and RAEFHI programs the tAFSET and tBESET delay range of the controller. The
standard resistor value selected for RAEFHI is 8.25 kΩ.
备注
tAFSET and tBESET can be programmed between 32 ns to 1100 ns.
The voltage at the ADELEF pin of the controller (VADELEF) needs to be set with RAEF based on the following
conditions.
• If tAFSET < 170 ns set VADEL = 0.2 V, tABSET can be programmed between 32 ns and 170 ns.
• If tABSET > or = 170 ns set VADEL = 1.7 V, tABSET can be programmed between 170 ns and 1100 ns.
Based on VADELEF selection, calculate RAEF
:
R
AEFHI ´ VADELEF
RAEF
=
» 4.25kW
5V - VADELEF
(138)
The closest standard resistor value for RAEF is 4.22 kΩ.
Recalculate VADELEF based on resistor divider selection:
5V ´RAEF
VADELEF
=
= 1.692V
R
AEFHI + RAEF
(139)
(140)
The following equation was used to program tAFSET and tBESET by properly selecting resistor REF.
t
(
´0.5 - 4ns
2.65V - V
)´ (
´1.32 ´103
ADELEF
)
1
AFSET
REF
=
´
» 14.1kW
ns
5
1A
The standard resistor value selected for REF is 14 kΩ.
Resistor RTMIN programs the minimum on time (tMIN) that the UCC28951 (Pin 9) can demand before entering
burst mode. If the UCC28951 controller tries to demand a duty cycle on time of less than tMIN the power supply
goes into burst mode operation. For this design set the minimum on-time (tMIN) to 75 ns.
Set the minimum on-time by selecting RTMIN
:
tMIN
RTMIN
=
» 12.7kW
5.92
(141)
(142)
The standard resistor value for RTMIN is 13 kΩ.
A resistor from the RT pin to ground sets the converter switching frequency calculated in 方程式142.
Ω Hz
V
6
2.5 × 10
f
×
Ω
V
R
=
−
× V
− 2.5 V = 60 kΩ
REF
T
SW
2
Copyright © 2023 Texas Instruments Incorporated
English Data Sheet: SLUSDB2
66
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