MP2269 - 30V, 1A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MP2269 Rev. 1.0

www.MonolithicPower.com

13

10/11/2016

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Soft Start (SS)

Soft start (SS) is implemented to prevent the

converter output voltage from overshooting

during start-up. When the soft-start period

begins, an internal current

source begins

charging the external soft-start capacitor. After

the soft-start voltage (SS) charges higher than

the internal offset voltage (typically 600mV),

than the internal reference (REF), the error

amplifier

uses

(SS -

calculated with Equation (2):

ref

Tss(ms) Iss( A)

Css(nF)

1.125 V (V)







(2)

Pre-Bias Start-Up

The MP2269 has been designed for monotonic

start-up into pre-biased loads. If the output is

pre-biased to a certain voltage during start-up,

the BST voltage is refreshed and charged. The

voltage on the soft-start capacitor is charged as

well. If the BST voltage exceeds its rising

threshold voltage, and the soft-start capacitor

voltage

exceeds

the

part

begins working normally.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from running away thermally. When the

silicon die temperature is higher than its upper

threshold, the power MOSFETs are shut down.

When the temperature is lower than its lower

threshold, thermal shutdown is removed, and

the chip is enabled again.

Current Comparator and Current Limit

The power MOSFET current is accurately

sensed via a current sense MOSFET. The

current is then fed to the high-speed current

comparator for current-mode control purposes.

The current comparator uses this sensed

current as one of its inputs. When the HS-FET

is turned on, the comparator is first blanked

until the end of the turn-on transition to avoid

noise. Then, the comparator compares the

the comparator outputs low to turn off the HS-

FET.

The maximum current of the internal power

MOSFET is limited cycle-by-cycle internally.

Hiccup Protection

When the output is shorted to ground, causing

the output voltage to drop below 70% of its

nominal output, the IC shuts down momentarily

and begins discharging the soft-start capacitor.

The IC restarts with a full soft start when the

soft-start capacitor is fully discharged. This

hiccup process is repeated until the fault is

removed.

Start-Up and Shutdown

If both VIN and EN are higher than their

respective thresholds, the chip starts up. The

reference block starts first, generating a stable

reference voltage and current, and then the

internal regulator is enabled. The regulator

provides a stable supply for the rest of the

circuitries.

While the internal supply rail starts up, an

internal timer holds the power MOSFET off for

about 50µs to blank the start-up glitches. When

the soft-start block is enabled, it first holds its

SS output low to ensure that the rest of the

circuitries are ready and then ramps up slowly.

Three events can shut down the chip: EN low,

VIN

low,

and

thermal

shutdown.

In

the

shutdown procedure, the signaling path is

and the internal supply rail are then pulled down.

The floating driver is not subject to this

shutdown command, but its charging path is

disabled.

Power Good (PG) Output

The MP2269 has an open-drain pin as the

power good indicator (PG). PG can indicate

under-voltage (UV) and over-voltage (OV). Pull

PG up to VCC through a 100k

Ω resistor. At the

an internal MOSFET pulls PG down to ground.