LTM4600HVMPV View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4600HVMPV' PDF : 24 Pages View PDF
LTM4600HV
applications information
After the controller has been started and given adequate
time to charge up the output capacitor, CSS is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging CSS. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
The over-current protection timer requires the soft-start
timing capacitor CSS be made large enough to guarantee
that the output is in regulation by the time CSS has reached
the 4V threshold. In general, this will depend upon the size
of the output capacitance, output voltage and load current
characteristic. A minimum external soft-start capacitor
can be estimated from:
CSS
_
EXT
+
1000pF
>
COUT • VOUT
10kV
Generally 0.1µF is more than sufficient.
Since the load current is already limited by the current
mode control and current foldback circuitry during a
short-circuit, over-current latchoff operation is NOT always
needed or desired, especially the output has large amount
of capacitance or the load draw huge current during start
up. The latchoff feature can be overridden by a pull-up
current greater than 5µA but less than 80µA to the RUN/
SS pin. The additional current prevents the discharge of
CSS during a fault and also shortens the soft-start period.
Using a resistor from RUN/SS pin to VIN is a simple solu-
tion to defeat latchoff. Any pull-up network must be able to
maintain RUN/SS above 4V maximum latchoff threshold
and overcome the 4µA maximum discharge current. With a
pull-up resistor, the delay before starting is approximately:
( ) tDELAY = –RRUN/SS • CSS _EXT + 1000pF
•
⎛
lnâŽâŽœâŽœ1−
VIN
+
1.5V
(1.2µA • RRUN/SS
)
⎞
⎠⎟⎟
Figure 3 shows a conceptual drawing of VRUN during
startup and short-circuit.
VRUN/SS
4V
3.5V
3V
1.5V
SHORT-CIRCUIT
LATCH ARMED
SOFT-START
CLAMPING
OF IL RELEASED
t
OUTPUT
OVERLOAD
SHORT-CIRCUIT
LATCHOFF
HAPPENS
VO
75%VO
SWITCHING
STARTS
Figure 3. RUN/SS Pin Voltage During Startup and
Short-Circuit Protection
t
4600hv F03
VIN
RRUN/SS
VIN
LTM4600HV
RUN/SS
PGND SGND
RECOMMENDED VALUES FOR RUN/SS
VIN
4.5V TO 5.5V
10.8V TO 13.8V
24V TO 28V
RRUN/SS
50k
150k
500k
4600hv F04
Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up
Resistor to VIN
For more information www.linear.com/LTM4600HV
4600hvfe
13
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