MP5077 – 5.5V, 7A, LOW RDSON LOAD SWITCH WITH PROGRAMMABLE CURRENT LIMIT
MP5077 Rev. 1.01 www.MonolithicPower.com 11
6/23/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP5077 is designed to limit the in-rush
current to the load thereby limiting the
backplane’s voltage drop and the slew rate of
the voltage to the load. It provides an integrated
solution to monitor the input voltage, output
voltage and output current to eliminate the need
for an external current power MOSFET, and
current switch device.
Enable
When input voltage is greater than the under-
voltage lockout threshold (UVLO), typically 0.5V,
MP5077 can be enabled by pulling EN pin to
higher than 1.5V. Pulling down to ground or
floating will disable MP5077.
Current Limit
The MP5077 provides a constant current limit
that can be programmed by an external resistor.
Once the device reaches its current limit
threshold, the internal circuit regulates the gate
voltage to hold the current in the power FET
constant. The typical response time is about
20µs and the output current may have a small
overshoot during this time period.
The pre-set current limit value can be
calculated by below equation:
I
Limit
= (1÷R
Limit
)×S (1)
S is the current sense ratio of MP5077, and this
value is typically 82000 in V
IN=3.6V. The other
values of S please refer the curves in typical
performance characteristics.
If the current limit block starts to regulate the
output current, the power loss on power
MOSFET will cause the IC temperature rise. If
the junction temperature rose to high enough, it
will trigger thermal shutdown. After thermal
shutdown happened, it will disable the output
until the over temperature fault remove. The
over temperature threshold is 155
o
C and
hysteresis is 30
o
C.
Short-Circuit Protection
If the load current increases rapidly due to a
short circuit, the current may exceed the current
limit threshold by a lot before the control loop
can respond. If the current reaches an internal
secondary current limit level (typical 13A), a fast
turn-off circuit activates to turn off the power
FET. This limits the peak current through the
switch to limit the input voltage drop. The total
short circuit response time is about 200ns. If
fast off works, it will keep off the power FET for
80μs. After that time period, it will re-turn on
power FET. If the part is still in short-circuit
condition. MP5077 will reduce the current limit
to 2/3 of pre-set value, and hold it until the part
is so hot and thermal shutdown. After the short-
circuit condition removed, the current limit will
recover to the pre-set value automatically.
Output Discharge
MP5077 has output discharge function. This
function can discharge the Vo by internal pull
down resistance when IC EN disabled or V
CC
shutdown and the load is very light.
Soft-Start
A capacitor connected to the SS pin determines
the soft-start time. There is an internal 9μA
constant current source charge SS cap and
ramps up the voltage on the SS pin. The output
voltage rises at 3 times the slew rate to SS
voltage.
The soft-start time can be calculated by below
equation:
(uA)I
(nF)C(V)V
3
1
(ms)T
SS
SSOUT
SS
⋅
×=
(2)
T
SS
is the soft-start time, I
SS
is internal 9μA
constant current, C
SS
is external soft-start cap.
The suggestion minimum SS cap should be
bigger than 4.7nF. If the SS pin is floated or SS
cap is too small, the Vout rising time will be just
limited by power MOS charge time.
