LTC4253AIGN-ADJ Linear Technology, LTC4253AIGN-ADJ Datasheet - Page 19

• Manufacturer Part Number: LTC4253AIGN-ADJ
• Description: IC,Power Control/Management,CMOS,SSOP,20PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC4253AIGN-ADJ

• Family Name: LTC4253A-ADJ
• Package Type: SSOP N
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Product Depth (mm): 3.99mm
• Product Height (mm): 1.5mm
• Mounting: Surface Mount
• Pin Count: 20
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
SENSE exceeds 50mV, the CB comparator activates the
200µA TIMER pull-up. At 60mV the ACL amplifier servos
the MOSFET current, and at 200mV the FCL comparator
abruptly pulls GATE low in an attempt to bring the MOSFET
current under control. If any of these conditions persists
long enough for TIMER to charge C
the LTC4253A-ADJ shuts down and pulls GATE low.
If the SENSE pin encounters a voltage greater than V
the ACL amplifier will servo GATE downwards in an
attempt to control the MOSFET current. Since GATE over-
drives the MOSFET in normal operation, the ACL amplifier
needs time to discharge GATE to the threshold of the
MOSFET. For a mild overload the ACL amplifier can control
the MOSFET current, but in the event of a severe overload
the current may overshoot. At SENSE = 200mV the FCL
comparator takes over, quickly discharging the GATE pin
to near V
amplifier takes over. All the while TIMER is running. The
effect of FCL is to add a nonlinear response to the control
loop in favor of reducing MOSFET current.
Owing to inductive effects in the system, FCL typically
overcorrects the current limit loop, and GATE under-
shoots. A zero in the loop (resistor R
gate capacitor) helps the ACL amplifier to recover.
SHORT-CIRCUIT OPERATION
Circuit behavior arising from a load side low impedance
short is shown in Figure 5. Initially the current overshoots
the analog current limit level of V
as the GATE pin works to bring V
The overshoot glitches the backplane in the negative direc-
tion and when the current is reduced to 60mV/R
backplane responds by glitching in the positive direction.
TIMER commences charging C
current limit loop maintains the fault current at 60mV/R
which in this case is 5A (trace 2). Note that the backplane
voltage (trace 1) sags under load. Timer pull-up is accel-
erated by V
PWRGD signals pull high, the load current drops to zero
and the backplane rings up to over 100V. The transient
associated with the GATE turn-off can be controlled with
a snubber to reduce ringing and a transient voltage
EE
OUT
potential. FCL then releases, and the ACL
. When C
U
T
reaches 4V, GATE turns off, the
U
T
GS
(trace 4) while the analog
SENSE
T
under control (trace 3).
to 4V (see Equation 4),
W
C
= 200mV (trace 2)
in series with the
U
S
, the
ACL
S
,
,
–48V RTN
suppressor (such as Diodes Inc. SMAT70A), to clip off
large spikes. The choice of RC for the snubber is usually
done experimentally. The value of the snubber capacitor
is usually chosen between 10 to 100 times the MOSFET
C
tween 3Ω to 100Ω.
A low impedance short on one card may influence the
behavior of others sharing the same backplane. The initial
glitch and backplane sag as seen in Figure 5 trace 1, can
rob charge from output capacitors on the adjacent card.
When the faulty card shuts down, current flows in to
refresh the capacitors. If LTC4253A-ADJs are used by the
other cards, they respond by limiting the inrush current to
a value of V
will recharge long before C
MOSFET SELECTION
The external MOSFET switch must have adequate safe
operating area (SOA) to handle short-circuit conditions
until TIMER times out. These considerations take prece-
dence over DC current ratings. A MOSFET with adequate
SOA for a given application can always handle the required
current but the opposite may not be true. Consult the
manufacturer’s MOSFET datasheet for safe operating area
and effective transient thermal impedance curves.
OSS
200mV
SENSE
TIMER
GATE
50V
10V
Figure 5. Output Short-Circuit Behavior of LTC4253A-ADJ
5V
. The value of the snubber resistor is typically be-
ACL
/R
S
ONSET OF OUTPUT
SHORT CIRCUIT
FAST CURRENT LIMIT
SUPPLY RING OWING
TO CURRENT OVERSHOOT
ANALOG CURRENT LIMIT
. If C
T
is sized correctly, the capacitors
0.5ms/DIV
LTC4253A-ADJ
T
times out.
C
TIMER
TO MOSFET TURN-OFF
SUPPLY RING OWING
RAMP
LATCH OFF
4253A F05
19
TRACE 1
TRACE 2
TRACE 3
TRACE 4
4253a-adjf