10
LTC1642
1642fb
APPLICATIO S I FOR ATIO
U
U
U
an internal servo loop adjusts the GATE pin voltage such
that Q1 acts as a constant current source. The voltage limit
across R2 increases as the output charges; this foldback
in the current limit helps to even out Q1s power dissipa-
tion. The output is sensed at the FB pin. When FB is
grounded, the sense voltage is limited to 26mV. When FB
is greater than 0.7V, the limit is 56mV and the full depen-
dence is shown in Figure 3.
When the sense resistor voltage is 3mV below its limit, the
circuit breaker timer starts. Once BRK TMR reaches its
threshold, the circuit breaker opens, the GATE pin is pulled
to ground (cutting off Q1) and FAULT is asserted.
The parameter V
CB
specified in the DC electrical character-
istics refers to the voltage difference between the V
CC
and
SENSE pins needed to start the circuit breaker timer. The
limiting value maintained by the servo loop is 3mV higher
than V
CB
.
Should the sense resistor voltage drop below its limit
before the timer trips, the GATE voltage begins ramping
back up immediately and the BRK TMR pin returns to
ground. However, due to the slow gate ramp, Q1 continues
to dissipate substantial power for some time. Connecting
R10 in series with timing capacitor C4 (as shown in
Figure 1) ensures that the circuit breaker trips in the event
of repetitive, but brief, load shorts. The delay before the
circuit breaker opens is:
t
BRKTMR
= C4 (61k& R10).
Once the circuit breaker trips, GATE and FAULT remain at
ground until the chip is restarted. To restart, hold the ON
pin low for at least 2祍 and FAULT will go high. Then take
ON high again and the GATE will ramp up after a system
timing cycle. Or, configure the LTC1642 to restart itself
after the circuit breaker trips by connecting FAULT to the
ON pin, as shown in the next section.
The servo loop controlling Q1 during current limit has a
unity-gain frequency of about 125kHz. In Figure 1, R4 and
C2 provide compensation. To ensure stability the product
1/(2 " ?" R4 " C2) should be kept below the unity-gain
frequency, and C2 should be more than Q1s input capaci-
tance C
ISS
. A good starting point for C2 is 0.047礔 and R4
is 330&. Keep R4 e 100&.
Typical waveforms during a load short to ground are
shown in Figure 4. The load is shorted to ground at time 1.
The GATE voltage drops until the load current equals its
maximum limit, and the circuit breaker timer starts. The
short is cleared at time 2, before the timer trips. The BRK
TMR pin returns to ground, and the GATE voltage begins
ramping up. At time 3 the load is shorted again and at time
4 the timer trips, pulling the GATE to ground and asserting
FAULT. Although the short is cleared at time 5, FAULT
doesnt go high until the ON pin is pulled low at time 6. At
time 7 ON goes high and the system timer starts. When it
trips at time 8 the GATE voltage begins ramping.
To disable current limit and electronic circuit breaker
protection, tie the SENSE pin to V
CC
, the BRK TMR pin to
GND and omit compensating resistor R4.
FB PIN VOLTAGE (mV)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8 0.9 1
1642 F03
70
60
50
40
30
20
10
0
Figure 3. Foldback Current Limit
Figure 4. Current Limit and Circuit Breaker Timing
40ms/DIV
1642 F04
t
1
t
2
t
3
t
4
t
5
t
7
t
6
t
8
RST TMR
2V/DIV
ON
20V/DIV
FAULT
20V/DIV
BRK TMR
2V/DIV
V
OUT
20V/DIV
GATE
20V/DIV
I
LOAD
5A/DIV
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