LT1976/LT1976B
17
1976bfg
APPLICATIO S I FOR ATIO
WUUU
Short-Circuit Considerations
The LT1976 is a current mode controller. It uses the V
C
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
this peak current is reached. The internal clamp on the V
C
node, nominally 2.2V, then acts as an output switch peak
current limit. This action becomes the switch current limit
specification. The maximum available output power is
then determined by the switch current limit.
A potential controllability prod}m could occur under short-
circuit conditions. If the power supply output is short
circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, V
C
, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V
C
. However, there is finite response
time involved in both the current comparator and turn-off
of the output switch. These result in a minimum on time
t
ON(MIN).
When combined with the large ratio of V
IN
to
(V
F
+ I • R), the diode forward voltage plus inductor I • R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
ft
VIR
V
ON
F
IN
•
•
≤
+
where:
f = switching frequency
t
ON
= switch on time
V
F
= diode forward voltage
V
IN
= Input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at I
PK
but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1976 clock frequency
of 200kHz, a V
IN
of 40V and a (V
F
+ I • R) of say 0.7V, the
maximum t
ON
to maintain control would be approximately
90ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
to allow the current in the inductor to drop to a sufficiently
low value such that the current doesn’t continue to ratchet
higher. When the FB pin voltage is abnormally low thereby
indicating some sort of short-circuit condition, the oscil-
lator frequency will be reduced. Oscillator frequency is
reduced by a factor of 4 when the FB pin voltage is below
0.4V and increases linearly to its typical value of 200kHz at
a FB voltage of 0.95V (see Typical Performance Character-
istics). In addition, if the current in the switch exceeds 1.5
• I
PK
current demanded by the V
C
pin, the LT1976 will skip
the next on cycle effectively reducing the oscillator fre-
quency by a factor of 2. These oscillator frequency reduc-
tions during short-circuit conditions allow the LT1976 to
maintain current control.
SOFT-START
For applications where [V
IN
/(V
OUT
+ V
F
)] ratios > 10 or
large input surge currents can’t be tolerated, the LT1976
soft-start feature should be used to control the output
capacitor charge rate during start-up, or during recovery
from an output short circuit thereby adding additional
control over peak inductor current. The soft-start function
limits the switch current via the V
C
pin to maintain a
constant voltage ramp rate (dV/dt) at the output capacitor.
A capacitor (C1 in Figure 2) from the C
SS
pin to the
regulated output voltage determines the output voltage
ramp rate. When the current through the C
SS
capacitor
exceeds the C
SS
threshold (I
CSS
), the voltage ramp of the
output capacitor is limited by reducing the V
C
pin voltage.
The C
SS
threshold is proportional to the FB voltage (see
Typical Performance Characteristics) and is defeated for
FB voltages greater than 0.9V (typical). The output dV/dt
can be approximated by:
dV
dt
I
C
CSS
SS
=
but actual values will vary due to start-up load conditions,
compensation values and output capacitor selection.
