MP2233 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP2233 Rev. 1.01

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11

12/6/2012

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enough hysteresis to allow for any input supply

variations.

Figure 3: Adjustable UVLO

External Soft-Start

The

MP2233

employs

a

soft-start

(SS)

mechanism to ensure smooth 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 switching

of both 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

CnF

VV

τ×

μ

=

(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 MP2233 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

MP2233 enters hiccup mode to periodically

restart the part. This protection mode is

especially useful when the output is dead-

shorted to ground. The average of short-circuit

current falls to alleviate thermal issues and to

protect the regulator. The MP2233 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 capacitor is strongly recommended to

reduce SW voltage spikes.

R3

Figure 4: Internal Bootstrap Charging Circuit