Datasheet

+
-
2PA
7PA
+
-
Q
Q
R
S
S>R
1.2V/
1.05V
ON/OFF
comparator
+
-
S/S level
S/S buffer
ON: 2.4PA source
Fault: 5.5PA sink
ON/SSx
ONx
fault
disable
V
ss
= 1.5
V
o
V
in
+ 1
¹
·
©
§
C
ss
=
I
ss
x t
ss
V
ss
LM5642, LM5642X
SNVS219K –JUNE 2003–REVISED APRIL 2013 www.ti.com
OPERATING DESCRIPTIONS
SOFT START
The ON/SS1 pin has dual functionality as both channel enable and soft start control. Referring to the soft start
block diagram is shown in Figure 25, the LM5642 will remain in shutdown mode while both soft start pins are
grounded.
In a normal application (with a soft start capacitor connected between the ON/SS1 pin and SGND) soft start
functions as follows: As the input voltage rises (note, Iss starts to flow when VIN ≥ 2.2V), the internal 5V LDO
starts up, and an internal 2.4 µA current charges the soft start capacitor. During soft start, the error amplifier
output voltage at the COMPx pin is clamped at 0.55V and the duty cycle is controlled only by the soft start
voltage. As the SSx pin voltage ramps up, the duty cycle increases proportional to the soft start ramp, causing
the output voltage to ramp up. The rate at which the duty cycle increases depends on the capacitance of the soft
start capacitor. The higher the capacitance, the slower the output voltage ramps up. When the corresponding
output voltage exceeds 98% (typical) of the set target voltage, the regulator switches from soft start to normal
operating mode. At this time, the 0.55V clamp at the output of the error amplifier releases and peak current
feedback control takes over. Once in peak current feedback control mode, the output voltage of the error
amplifier will travel within a 0.5V and 2V window to achieve PWM control. See Figure 26.
The amount of capacitance needed for a desired soft-start time can be approximated in the following equation:
where
• I
ss
= 2.4 µA for one channel and 4.8µA if the channels are paralleled
• t
ss
is the desired soft-start time (1)
Finally,
(2)
During soft start, over-voltage protection and current limit remain in effect. The under voltage protection feature is
activated when the ON/SS pin exceeds the timeout threshold (3.4V typical). If the ON/SSx capacitor is too small,
the duty cycle may increase too rapidly, causing the device to latch off due to output voltage overshoot above the
OVP threshold. This becomes more likely in applications with low output voltage, high input voltage and light
load. A capacitance of 10 nF is recommended at each soft start pin to provide a smooth monotonic output ramp.
Figure 25. Soft-Start and ON/OFF
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