May 2003

7

MIC2145

MIC2145

Micrel

Functional Description

See

pre-designed circuits.

Overview

The MIC2145 is a 2.5W boost regulator with programmable

peak current limit and a constant off time. Quiescent current

for the MIC2145 is typically 200

µA when the switch is in the

off state. Efficiencies above 80% throughout most operating

conditions can be realized.

Regulation

Regulation is achieved by both of the comparators, which

regulate the inductor current and the output voltage by gating

the power MOSFET. Initially, power is applied to the SW and

VDD pins. When the part is enabled, the power MOSFET

turns on and current flows. When the current exceeds the

peak current limit threshold, the current limit comparator fires

the one-shot to turn off the power MOSFET for 1000ns and

resets the SR flip-fop. The current limit comparator continues

to cycle the power MOSFET on and off until the output voltage

trips the upper threshold of the feedback comparator, which

terminates the cycle. The cycle will begin again when the

output voltage drops below the lower hysteresis threshold of

the feedback comparator. The feedback comparator has a

typical hysteresis of 18mV. Due to the gain of the feedback

resistor divider, the voltage at V

167mV of hysteresis for 10V output at 2.4V V

reduced by adding a feed-forward capacitor, C

Time 20

µs

V

OUT

AC Couple

(100mV/div)

V

SW

(5V/div)

I

INDUCTOR

(500mA/div)

Figure 2. Typical Regulator Waveforms

Output

The maximum output voltage is limited by the voltage capa-

bility of the output switch. Output voltages of up to 16V can be

achieved with the boost circuit. Higher output voltages re-

quire a flyback configuration.

Peak Current Limit

The peak current limit is externally set with a resistor. The

peak current range is from 420mA to 2A. There is a minimum

resistor value for R

value selections, see the

“Typical Characteristics: Peak

Soft Start

The MIC2145 has a built in soft start that controls the rise time

of the output voltage and the peak current limit threshold

during start up.

Time 200

µs

V

OUT

(5V/div)

V

EN

(2V/div)

V

SW

(5V/div)

I

INDUCTOR

(500mA/div)

V

V

R

C

Figure 3. Typical Soft Start Waveforms

Thermal Shutdown

Built-in thermal protection circuitry turns off the power MOSFET

when the junction temperature exceeds about 150

°C.