MIC4721 datasheet(8/19 Pages) MICREL

8 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

Pin Descriptions

VIN

Two pins for VIN provide power to the source of the

internal P-channel MOSFET along with the current

limiting sensing. The VIN operating voltage range is from

2.7V to 5.5V. Due to the high switching speeds, a 10µF

capacitor should be placed close to VIN and power

ground (PGND) at each pin for bypassing. Please refer

to the section on layout recommendations.

BIAS

The bias (BIAS) provides power to the internal reference

and control sections of the MIC4721. A 10

Ω resistor

from VIN to BIAS and a 0.1µF from BIAS to SGND is

required.

The enable pin provides a logic level control of the

output. At a low level, the output is shut off and supply

current at the VIN pin is greatly reduced (typically <2µA).

A high level turns the output on. The enable pin must not

be driven above the supply voltage.

The feedback pin (FB) connects to the negative input of

the internal error amplifier. The pin is connected to an

external resistor divider, which sets the output voltage.

A feedforward capacitor across the upper resistor in the

divider is recommended for most designs. To reduce

current draw, a 10K feedback resistor is recommended

from the output to the FB pin (R1). The large resistor

value and the parasitic capacitance of the FB pin can

cause a high frequency pole that can reduce the overall

system

phase

margin.

placing

feedforward

capacitor, these effects can be significantly reduced.

The switch (SW) pin connects directly to the inductor

and provides the switching current necessary to operate

in PWM mode. Due to the high speed switching on this

pin, the switch node should be routed away from

sensitive nodes. This pin also connects to the cathode of

the free-wheeling diode.

PGOOD

The power good pin pulls low to indicate the output

voltage is within its regulation range. When power good

is low, then the output voltage is within ±10% of the set

regulation voltage. For output voltages greater or less

than 10%, the PGOOD pin is high. This should be

connected to the input supply through a pull up resistor.

A delay can be added by placing a capacitor from

PGOOD-to-ground.

PGND

Power ground (PGND) is the ground path for the

MOSFET drive current. The current loop for the power

ground should be as small as possible and separate

from the Signal ground (SGND) loop. Refer to the

section on layout recommendations for more details.

SGND

Signal ground (SGND) is the ground path for the biasing

and control circuitry. The current loop for the signal

ground should be separate from the power ground

(PGND)

loop.

Refer

the

section

layout

recommendations for more details.

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