LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 14

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
A potential controllability problem 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
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
time involved in both the current comparator and turn-off
of the output switch. This results in a minimum on time
t
(V
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
where:
If this condition is not observed, the current will not be
limited at I
higher value. Using the nominal LT3437 clock frequency
of 200kHz, a V
maximum t
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 does not continue to
ratchet higher. When the FB pin voltage is abnormally low,
thereby indicating some sort of short-circuit condition,
the oscillator frequency will be reduced. Oscillator fre-
quency is reduced by a factor of 10 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
Characteristics). These oscillator frequency reductions
during short-circuit conditions allow the LT3437 to main-
tain current control
14
ON(MIN).
F
f = switching frequency
t
V
V
I • R = inductor I • R voltage drop
f t
ON
+ I • R), the diode forward voltage plus inductor I • R
F
IN
•
= diode forward voltage
= switch on time
ON
= Input voltage
When combined with the large ratio of V
≤
PK
ON
V
F
but will cycle-by-cycle ratchet up to some
to maintain control would be approximately
IN
V
+
IN
of 40V and a (V
I R
•
U
C
. However, there is finite response
U
F
+ I • R) of say 0.7V, the
W
U
C
, to its
IN
to
SOFT-START
For applications where [V
surge currents cannot be tolerated, the LT3437 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
voltage ramp rate (dV/dt) at the output capacitor. A capaci-
tor (C1 in Figure 2) from the C
voltage determines the output voltage ramp rate. When
the current through the C
threshold (I
is limited by reducing the V
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:
but actual values will vary due to start-up load conditions,
compensation values and output capacitor selection.
Burst Mode OPERATION
To enhance efficiency at light loads, the LT3437 automati-
cally switches to Burst Mode operation which keeps the
output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst
Mode operation, the LT3437 delivers short bursts of
dV
dt
1V/DIV
=
V
OUT
I
C
CSS
SS
CSS
V
C
I
LOAD
IN
OUT
C
SS
= 12V
), the voltage ramp of the output capacitor
= 100µF
= GND
= 200mA
Figure 4. V
C
SS
= 0.01µF
IN
C
SS
/(V
1ms/DIV
pin voltage. The C
C
SS
OUT
OUT
capacitor exceeds the C
pin to maintain a constant
pin to the regulated output
dV/dt
+ V
F
)] >10 or large input
C
SS
= 0.1µF
3437 F04
SS
threshold
3437f
SS