Datasheet

( )
SS
CL(on _ master )
C 4.65 V 3.7 V
T
20 A
´ -
=
m
( )
SS
CL(on _ slave)
C 4.65 V 3.7 V
T
25 A
´ -
=
m
( )
SS
CL(off _ m aster )
C 3.6 V 0.55 V
T
2.5 A
´ -
=
m
( )
SS
CL(off _ slave)
C 3.6 V 0.55 V
T
4.9 A
´ -
=
m
UCC28950
www.ti.com SLUSA16B –MARCH 2010–REVISED OCTOBER 2011
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 which is defined as:
(17)
(18)
Thus, if the soft-start capacitor C
SS
= 100 nF is selected, then the T
CL(on)
time will be 5 ms.
To calculate the hiccup off time T
CL(off)
before the restart, the following Equation 19 or Equation 20 needs to be
used:
(19)
(20)
With the same soft start capacitor value 100 nF, the off time before the restart is going to be 122 ms. Notice, that
if the over current condition happens 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 R
DS(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 default parameters 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 Figure 18. In this example the cycle-by-cycle current limit lasts about 5 ms followed by 122
ms of off time.
Similar to the over current condition, the hiccup mode with the restart can be overridden by the user if a pull up
resistor is connected between the SS and VREF pins. If the pull up current provided by the resistor exceeds 2.5
µA, then the controller remains in the latch off mode. In this case, an external soft-start capacitor value should be
calculated with the additional pull-up current taken into account. The latch off mode can be reset externally if the
soft-start capacitor is forcibly discharged below 0.55 V or the V
DD
voltage is lowered below the UVLO threshold.
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