Micrel

MIC2297

May 2005

9

M9999-050305

(408) 955-1690

Functional Description

The MIC2297 is a constant frequency, PWM current mode

boost regulator. The MIC2297 uses peak current mode

control. The block diagram is shown above. The MIC2297

is composed of an oscillator, slope compensation ramp

generator, and a 1.2A bipolar output transistor. The

oscillator generates a 600kHz 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.

the external sense resistor and amplifies the error between

the detected signal and the 200mV reference voltage. The

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. The LED current is set by the feedback

resistor:

FB

LED

R

mV

I

200

=

The Enable pin shuts down the output switching and

disables control circuitry to reduce input current-to-leakage

levels. Enable pin input current is zero at zero volts.

DC-to-DC PWM Boost Conversion

The MIC2297 is a constant-frequency boost converter. It

operates by taking a DC input voltage and regulating a

higher DC output voltage. Figure 2 shows a typical circuit.

Boost regulation is achieved by turning on an internal

switch, which draws current through the inductor (L1).

When the switch turns off, the inductor’s magnetic field

collapses. This causes the current to be discharged into

the output capacitor through an external Schottky diode

(D1). Waveforms 1 and 2 show Output Voltage ripple, SW

Voltage, and Indicator Current for 5mA and 20mA LED

current respectively. Voltage regulation is achieved by

modulating the pulse width or pulse-width modulation

(PWM).

VIN

EN

SW

FB

MIC2297

-42BM L

0.47µF

/50V

6.8µH-22µH

10

OVP

PGND

1µF

1-Cell

Li I on

3V to 4.2V

BRT

REF

AGND

COMP

1µF

0.1µF

Figure 2. Typical Application Circuit

Duty Cycle Considerations

Duty cycle refers to the switch on-to-off time ratio and can

be calculated as follows for a boost regulator:

out

in

V

V

D

−

= 1

However, at light loads the inductor will completely

discharge before the end of a switching cycle. The current

in the inductor reaches 0A before the end of the switching

cycle. This is known as discontinuous conduction mode

(DCM). DCM occurs when:

2

peak

out

in

out

I

V

V

I

⋅

<

Where

(

)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

⋅

−

=

out

in

in

out

peak

V

V

f

L

V

V

I

In DCM, the duty cycle is smaller than in continuous conduction

mode. In DCM the duty cycle is given by:

in

in

out

out

V

V

V

I

L

f

D

)

(

2

−

⋅

⋅

⋅

⋅

=

The duty cycle required for voltage conversion should be

less than the maximum duty cycle of 95%. Also, in light

load conditions where the input voltage is close to the

output voltage, the minimum duty cycle can cause pulse

skipping. This is due to the energy stored in the inductor

causing the output to overshoot slightly over the regulated

output voltage. During the next cycle, the error amplifier

detects the output as being high and skips the following

pulse. This effect can be reduced by increasing the

minimum load or by increasing the inductor value.

Increasing the inductor value reduces peak current.