MP1498 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
shorted to ground. The average short-circuit
current falls to alleviate thermal issues and to
protect the regulator. The MP1498 exits hiccup
mode once the over-current condition is
removed.
Thermal Shutdown
Thermal shutdown prevents the chip from
operating at exceedingly high temperatures.
When the silicon die temperature exceeds
150°C, it shuts down the whole chip. When the
temperature drops below its lower threshold
(typically 130°C) the chip is enabled again.
Figure 3: Adjustable UVLO
External Soft-Start
The MP1498 employs
a
soft-start (SS)
mechanism to smooth the output during power-
up. When the EN pin goes high, an internal
current source (14μA) charges the SS
capacitor. The SS capacitor voltage overtakes
the REF voltage to the PWM comparator. The
output voltage smoothly ramps up with the SS
voltage. Once the SS voltage reaches the same
level as the REF voltage, it keeps ramping up
while VREF takes over the PWM comparator. At
this point, the soft-start finishes and the device
enters steady state operation.
Floating Driver and Bootstrap Charging
An external bootstrap capacitor powers the
floating power MOSFET driver. This floating
driver has its own UVLO protection with a rising
threshold of 2.2V and a hysteresis of 150mV.
The bootstrap capacitor voltage is regulated
internally by VIN through D1, M1, R3, C4, L1
and C2 (Figure 4). If (VIN-VSW) exceed 5V, U1
will regulate M1 to maintain a 5V BST voltage
across C4. A 20ꢀ resistor placed between SW
and BST cap is strongly recommended to
reduce SW spike voltage.
If the output is pre-biased to a certain voltage
during startup, the IC will disable the high-side
and low-side switches until the voltage on the
internal soft-start capacitor exceeds the sensed
output voltage at the FB pin.
The SS capacitor value can be determined as
follows:
R3
SS (ms) ISS (A)
CSS (nF)
(1)
VREF(V)
If
the
output
capacitors
have
large
capacitances, avoid setting a short SS time to
avoid hitting the current limit during SS. Use a
minimum value of 4.7nF if the output
capacitance value is larger than 330μF.
Figure 4: Internal Bootstrap Charging Circuit
Over-Current Protection and Hiccup
The MP1498 has a cycle-by-cycle over-current
limit that protects against the inductor current
peak value exceeding the set current limit
threshold. Under-voltage protection (UVP)
triggers if the FB voltage drops below the
under-voltage (UV) threshold—typically 50%
below the reference. Once UVP triggers, the
MP1498 enters hiccup mode to periodically
restart the part. This protection mode is
especially useful when the output is dead-
MP1498 Rev. 1.01
12/18/2012
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