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MIC2145

High Efficiency 2.5W Boost Converter

Micrel 

MIC2145

Functional Description

See “Application Information” for component selection and

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 200µA 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 VOUT experiences a typical

167mV of hysteresis for 10V output at 2.4V VDD. This can be

reduced by adding a feed-forward capacitor, CFF (See

“Output Voltage” section).

VOUT

AC Couple

(100mV/div)

Micrel, Inc.

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 RSET at lower VDD voltages. For resistor

value selections, see the “Typical Characteristics: Peak

Current Limit vs. RSET”.

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.

VEN

(2V/div)

VOUT

(5V/div)

VSW

(5V/div)

VIN = 3V

VOUT = 10V

RSET = 10k

CSS = 0.01µF

IINDUCTOR

(500mA/div)

Time 200µs

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.

VSW

(5V/div)

IINDUCTOR

(500mA/div)

Time 20µs

Figure 2. Typical Regulator Waveforms

April 2005

7

M9999-042205

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