Micrel, Inc.

MIC2605/6

August 2008

8

M9999-080108-B

Functional Description

The MIC2605/6 is a constant frequency, PWM current

mode boost regulator. The block diagram is shown in

Figure 1. The MIC2605/6 is composed of an oscillator,

slope compensation ramp generator, current amplifier,

output transistor. The oscillator generates a 1.2MHz/

2MHz clock. The clock’s two functions are to trigger the

PWM generator that turns on the output transistor and to

reset the slope compensation ramp generator. The

current amplifier is used to measure the switch current

by amplifying the voltage signal from the internal sense

resistor. The output of the current amplifier is summed

with the output of the slope compensation ramp

generator. This summed current-loop signal is fed to one

of the inputs of the PWM generator.

through the external feedback resistors and amplifies the

error between the detected signal and the 1.25V

reference voltage. The output of the gm error amplifier

provides the voltage-loop signal that is fed to the other

input of the PWM generator. When the current-loop

signal exceeds the voltage-loop signal, the PWM

generator turns off the bipolar output transistor. The next

clock

period

initiates

the

next

switching

cycle,

maintaining the constant frequency current-mode PWM

control.

Pin Description

VIN

VIN provides power to the MOSFETs for the switch

mode regulator section. Due to the high switching

speeds, a 1µF capacitor is recommended close to VIN

and the power ground (PGND) pin for bypassing. Please

refer to layout recommendations.

VDD

The VDD pin supplies the power to the internal power to

the control and reference circuitry. The VDD is powered

from VIN. A small 0.1µF capacitor is recommended for

bypassing.

EN

The enable pin provides a logic level control of the

output. In the off state, supply current of the device is

greatly reduced (typically <0.1µA). Also, in the off state,

the output drive is placed in a "tri-stated" condition,

where bipolar output transistor is in an “off” or non-

conducting state. Do not drive the enable pin above the

supply voltage.

SS

The SS pin is the soft start pin which allows the

monotonic buildup of output when the MIC2605/6 comes

up during turn on. The SS pin gives the designer the

flexibility to have a desired soft start by placing a

capacitor SS to ground. A 0.1µF capacitor is used for in

the circuit.

FB

The feedback pin (FB) provides the control path to

control the output. For fixed output controller output is

directly connected to feedback (FB) pin.

SW

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 and high

voltage associated with this pin, the switch node should

be routed away from sensitive nodes.

PGND

Power ground (PGND) is the ground path for the high

current PWM mode. The current loop for the power

ground should be as small as possible and separate

from the Analog ground (AGND) loop. Refer to the layout

considerations for more details.

VOUT

VOUT pin is the cathode of pin of internal schottky

diode. This pin is connected to output cap. At least 1µF

cap is recommended very close to the VOUT pin and

PGND.