UCC28951
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ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
I
= −25 × 1 − D µA
(17)
DS follower
The soft-start capacitor value also determines the so-called hiccup mode off-time duration. The behavior of the
converter during different modes of operation, along with related soft-start capacitor charge and discharge
currents are shown in 图7-15.
Cycle-by-Cycle ILIM
SS Pin (V)
Normal
Operation
Soft Start
.
OFF Time Before Restart
SS Clamp Voltage
Pull Up Threshold
4.65
25 mA
Soft Restart
3.70
3.60
Fast Pull Up
by 1 kW Switch
IDS = (-25 x (1-D)+5) mA
IHCC = 2.5 mA
ISS=25 mA
Output Enable
Threshold
0.55
0.00
Output Pulses (D)
图7-15. Timing Diagram of Soft-Start Voltage VSS
The largest discharge current of 20 µA is when the duty cycle is close to zero. This current sets the shortest
operation time during the cycle-by-cycle current limit and is defined in 方程式18 and 方程式19
C
× 4.65 V − 3.7 V
SS
T
T
=
(18)
(19)
CL on_leader
20 µA
C
× 4.65 V − 3.7 V
SS
=
CL on_follower
25 µA
Thus, if the soft-start capacitor CSS = 100 nF is selected, then the TCL(on) time is 5 ms.
To calculate the hiccup off time TCL(off) before the restart, use 方程式20 or 方程式21.
C
× 4.65 V − 3.7 V
SS
T
T
=
(20)
(21)
CL off_leader
2.5 µA
C
× 4.65 V − 3.7 V
SS
=
CL off_follower
2.5 µA
With the same soft-start capacitor value at 100 nF, the off-time before the restart is 122 ms. If the overcurrent
condition occurs before the soft-start capacitor voltage reaches the 3.7-V threshold during start-up, the controller
limits the current but the soft-start capacitor continues to be charged. As soon as the 3.7-V threshold is reached,
the soft-start voltage is quickly pulled up to the 4.65-V threshold by an internal 1-kΩ RDS(on) switch and the
cycle-by-cycle current limit duration timing starts by discharging the soft-start capacitor. Depending on specific
design requirements, the user can override this default behavior by applying external charge or discharge
currents to the soft-start capacitor. The whole cycle-by-cycle current limit and hiccup operation is shown in 图
7-15. In this example, the cycle-by-cycle current limit lasts about 5 ms followed by 122 ms of off-time.
Similarly to the overcurrent condition, the hiccup mode with the restart can be disabled by the user if a pullup
resistor of 261 k Ω is connected between the SS and VREF pins. The controller remains in the latch-off mode if
an overcurrent condition occurs. In this case, calculate an external soft-start capacitor value with the additional
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English Data Sheet: SLUSDB2
34
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