UCC28951
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ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
7.3.4 Soft-Start and Enable (SS/EN)
The soft-start pin (SS/EN) is a multi-function pin used for the following operations:
• Closed-loop soft start with the gradual duty cycle increase from the minimum set by TMIN up to the steady-
state duty cycle required by the regulated output voltage.
• Setting hiccup mode conditions during cycle-by-cycle overcurrent limit.
• On/off control for the converter.
During the soft-start sequence, one of the voltages at the SS/EN or EA+ pins, whichever is lower (SS/EN –0.55
V) or EA+ voltage (see 节 7.2), sets the reference voltage for a closed feedback loop. Both SS/EN and EA+
signals are noninverting inputs of the error amplifier with the COMP pin being its output. Thus the soft-start time
always goes under the closed feedback loop and the voltage at COMP pin sets the duty cycle. The duty cycle
defined by the COMP pin voltage can not be shorter than TMIN pulse width set by the user. However, if the
shortest duty cycle is set by the cycle-by-cycle current limit circuit, then it becomes dominant over the duty cycle
defined by the COMP pin voltage or by the TMIN block.
The soft-start duration is defined by an external capacitor CSS, connected between the SS/EN pin and ground,
and the internal charge current that has a typical value of 25 µA. Pulling the soft-start pin externally below 0.55 V
shuts down the controller. The release of the soft-start pin enables the controller to start, and if there is no
current limit condition, the duty cycle applied to the output inductor gradually increases until it reaches the
steady-state duty cycle defined by the regulated output voltage of the converter. This increase happens when
the voltage at the SS/EN pin reaches and then exceeds by 0.55 V, the voltage at the EA+ pin. Thus for the given
soft-start time TSS, the CSS value can be defined by 方程式1 or 方程式2:
T
× 25 µA
SS
0.55 + V
C
=
(1)
(2)
SS leader
EA +
T
× 25 µA
SS
C
=
SS follower
20.6
20.6 − 0.55 − V
825 kΩ × ln
EA +
For example, in 方程式1, if the soft-start time TSS is 10 ms, and the EA+ pin is 2.5 V, then the soft-start capacitor
CSS is equal to 82 nF.
备注
If the converter is configured to operate in follower mode, connect a 825-kΩ (±5%) resistor from the
SS pin to ground.
7.3.5 Light-Load Power Saving Features
The UCC28951 offers four different light-load management techniques for improving the efficiency of a power
converter over a wide load current range.
1. Adaptive Delay,
a. ADEL, which sets and optimizes the dead-time control for the primary switches over a wide load current
range.
b. ADELEF, which sets and optimizes the delay-time control between the primary side switches and the
secondary side switches.
2. TMIN, sets the minimum pulse width as long as the part is not in current limit mode.
3. Dynamic synchronous rectifier on/off control in DCM Mode, For increased efficiency at light loads. The DCM
Mode starts when the voltage at CS pin is lower than the threshold set by the user. In DCM Mode, the
synchronous output drive signals OUTE and OUTF are brought down low.
4. Burst Mode, for maximum efficiency at very light loads or no load. Burst Mode has an even number of PWM
TMIN pulses followed by off time. Transition to the Burst Mode is defined by the TMIN duration set by the
user.
Copyright © 2023 Texas Instruments Incorporated
English Data Sheet: SLUSDB2
22
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