Datasheet

( )
SS
SS(master )
T 25 A
C
VNI 0.55
´ m
=
+
SS
SS(slave)
T
C
20.6
825K Ln
20.6 VNI 0.55
=
æ ö
´
ç ÷
- -
è ø
UCC28950
SLUSA16B –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
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 over current limit.
• On/off control for the converter.
During soft start, one of the voltages at the SS/EN or EA+ pins, whichever is lower (SS/EN - 0.55 V) or EA+
voltage (see Block Diagram), sets the reference voltage for a closed feedback loop. Both SS/EN and EA+ signals
are non-inverting inputs of the error amplifier with the COMP pin being its output. Thus the soft start always goes
under the closed feedback loop and the voltage at COMP pin sets the duty cycle. The duty cycle defined by
COMP voltage can not be shorter than TMIN pulse 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 COMP voltage or
by TMIN block.
The soft-start duration is defined by an external capacitor C
SS
, connected between SS/EN pin and ground, and
the internal charge current that has 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 happens when the voltage at the SS/EN pin
reaches and then exceeds the voltage at EA+ pin defined as VNI by 0.55 V. Thus for the given soft-start time
T
SS
, the C
SS
value can be defined by Equation 1 or Equation 2:
(1)
(2)
For example, in Equation 1, if the soft-start time T
SS
is selected to be 10 ms, and the VNI is 2.5 V, then the
soft-start capacitor C
SS
is equal to 82-nF.
NOTE
If the converter is configured in Slave Mode, make sure you place an 825-kΩ resistor from
SS pin to ground.
Light-Load Power Saving Mode
The UCD28950 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 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 duty cycle 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.
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