MP2235

– SYNCHRONOUS STEP-DOWN CONVERTER

MP2235 Rev. 1.0

www.MonolithicPower.com

11

5/16/2013

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Figure 3

Soft-Start

Adjust the soft-start time by connecting a

capacitor from SS pin to ground. When the soft-

start begins, an internal 11µA current source

charges the external capacitor. During soft-

start, the soft-start capacitor connects to the

non-inverting input of the error amplifier. The

soft-start period continues until the voltage on

the

soft-start

capacitor

exceeds

the

0.8V

reference. Then the non-inverting amplifier uses

the reference voltage takes as the input. Use

the following equation to calculate the soft-start

time:

SS

0.8V Css(nF)

t (ms)

11 A







Power Save Mode for Light Load Condition

The

MP2235

has

AAM

(Advanced

Asynchronous Modulation) power save mode

for light load. The AAM voltage is set at 0.6V

internally. Under the heavy load condition, the

high, the high-side power MOSFET turns on

set by the COMP voltage. The internal clock

Under the light load condition, the value of

clock is blocked, thus the MP2235 skips some

pulses for PFM (Pulse Frequency Modulation)

mode and achieves the light load power save.

Figure 4: Simplified AAM Control Logic

When the load current is light, the inductor peak

current is set internally which is about 0.9A for

Over-Current-Protection and Hiccup

The MP2235 has a cycle-by-cycle over-current

limit when the inductor current peak value

exceeds

the

set

current

limit

threshold.

below the Under-Voltage (UV) threshold

—

typically 50% below the reference. Once UV is

triggered, the MP2235 enters hiccup mode to

periodically restart the part. This protection

mode is especially useful when the output is

dead-shorted to ground, and greatly reduces

the average short circuit current to alleviate

thermal issues and protect the regulator. The

MP2235 exits the 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 reaches temperatures that

exceed 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. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

maintain a 5V BST voltage across C4. A 2

0Ω

resistor placed between SW and BST cap. is

strongly recommended to reduce SW spike

voltage.