MP4566―36V, 1MHz, 0.6A, STEP-DOWN CONVERTER WITH 35μA QUIESCENT CURRENT

MP4566 Rev. 1.01

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9

9/22/2011

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© 2011 MPS. All Rights Reserved.

Error Amplifier

The Error amplifier is composed of an internal

OP-AMP

with

an

R-C

feedback

network

connected between its output node (internal

COMP node) and its negative input node (FB).

When FB is lower than its internal reference

voltage (REF), the COMP output is then driven

higher by the OP-AMP, causing higher switch

peak current output hence more energy delivered

to the output. Vise versus.

When connecting to the FB pin, normally there is

serves also to control the gain of the error

amplifier along with the internal compensation R-

C network.

Internal Regulator

Most of the internal circuitries are powered on by

the 2.6V internal regulator. This regulator takes

VIN input and operates in the full VIN range.

When VIN is greater than 3.0V, the output of the

regulator is in full regulation. When VIN is lower,

the output degrades.

Enable Control

The MP4566 has a dedicated enable control pin

EN. With high enough VIN, the chip can be

enabled and disabled by EN pin. This is a HIGH

effective logic. Its trailing threshold is a consistent

1.2V. Its rising threshold is about 400mV higher.

When floating, EN pin is internally pulled up high

so the chip is automatically on.

When EN is pulled down to 0V, the chip is put

into the lowest shutdown current mode. When

EN is higher than zero but lower than its rising

threshold, the chip is still in shutdown mode but

the shutdown current increases slightly.

Under Voltage Lockout (UVLO)

VIN

Under

voltage

lockout

(UVLO)

is

implemented to protect the chip from operating at

insufficient supply voltage. The UVLO rising

threshold is about 4.0V while its trailing threshold

is a consistent 3.6V.

Internal Soft-start

Reference type soft-start is implemented to

prevent the converter output voltage from

overshooting during startup. When the chip starts,

the internal circuitry generates a soft-start voltage

(SS) ramping up from 0V at a slow pace set by

the soft-start time. When it is lower than the

internal reference REF, SS overrides the REF so

the error amplifier uses SS instead of REF as the

reference. When SS is higher than REF, REF

gains the control back.

SS is also associated with FB. Though SS can

be much lower than FB, it can only be slightly

higher than FB. If somehow FB is brought down,

SS follows to track FB. This function is designed

to accommodate the short-circuit recovery

situation. When the short circuit is removed, the

SS ramps up as if it is a fresh soft-start process.

This prevents output voltage overshoot.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip from thermally running away. When the

silicon die temperature is higher than its upper

threshold, it shuts down the whole chip. When

the temperature is lower than its lower threshold,

thermal shutdown is gone so the chip is enabled

again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor.

This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is about 2.4V with a threshold of

about 300mV. During this UVLO, the SS voltage

of the controller is reset to zero. When the UVLO

is removed, the controller follows soft-start

process.

The bootstrap capacitor is charged and regulated

to about 5V by the dedicated internal bootstrap

regulator. When the voltage between BST and

SW nodes is lower than its regulation, a PMOS

pass transistor connected from VIN to BST is

turned on. The charging current path is from VIN,

BST and then to SW. External circuit should

provide enough voltage headroom to facilitate the

charging.