MP1498 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1498 Rev. 1.01

www.MonolithicPower.com

11

12/18/2012

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© 2012 MPS. All Rights Reserved.

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

this point, the soft-start finishes and the device

enters steady state operation.

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:

SS

SS

SS

REF

(ms) I

( A)

C(nF)

V(V)







(1)

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.

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-

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.

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

will regulate M1 to maintain a 5V BST voltage

across C4. A 20Ω resistor placed between SW

and BST cap is strongly recommended to

reduce SW spike voltage.

R3

Figure 4: Internal Bootstrap Charging Circuit