Micrel, Inc.

MIC2298

October 2007

7

M9999-101707-B

Functional Description

The MIC2298 is a constant frequency, pulse-width-

modulated (PWM) peak current-mode step-up regulator.

The MIC2298 simplified control scheme is illustrated in

the block diagram in Figure 2. A reference voltage is fed

into the PWM engine where the duty cycle output of the

constant frequency PWM engine is computed from the

error, or difference, between the REF and FB voltages.

The PWM engine encompasses the necessary circuit

blocks to implement a current-mode boost switch-mode

power supply. The necessary circuit blocks include, but

are not limited to, a oscillator/ramp generator, slope

amplifier, PWM comparator, and drive logic for the

internal 3.5A bipolar power transistor.

Inside the PWM engine, the oscillator functions as a

trigger for the PWM comparator that turns on the bipolar

power transistor and resets the slope compensation

ramp generator. The current amplifier is used to

measure the power transistor’s current by amplifying the

voltage signal from the CS+ and CS- inputs from the

sense resistor connected to the emitter of the bipolar

power transistor. The output of the current amplifier is

summed with the output of the slope compensation ramp

generator where the result is connected to one of the

inputs of the PWM comparator.

through the external sense resistor and amplifies the

error between the detected voltage signal from the

feedback, or FB pin and the internal reference voltage.

loop signal that is fed to the other input of the PWM

comparator. When the current loop signal exceeds the

voltage loop signal the PWM comparator turns off the

power transistor. The next oscillator/clock period initiates

the next switching cycle, maintaining the constant

frequency current-mode PWM control. The maximum

white LED current is set by the feedback resistor (the

resistor connected from the feedback pin to ground):

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 MIC2298 is a constant-frequency boost converter. It

operates by taking a DC input voltage and regulating a

DC output voltage. Figure 3 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 4 and 5 show Input Voltage ripple,

Output Voltage ripple, SW Voltage, and Inductor Current

for 900mA LED current. Regulation is achieved by

modulating the pulse width i.e., pulse-width modulation

(PWM).

Figure 3. 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 slightly overshoot 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 also reduces the peak

current.