April 2005 7 M9999-042205
MIC2145 Micrel, Inc.
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 V
OUT
experiences a typical
167mV of hysteresis for 10V output at 2.4V V
DD
. This can be
reduced by adding a feed-forward capacitor, C
FF
(See
“Output Voltage” section).
Time 20µs
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
SET
at lower V
DD
voltages. For resistor
value selections, see the “Typical Characteristics: Peak
Current Limit vs. R
SET
”.
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 200µs
V
OUT
(5V/div)
V
EN
(2V/div)
V
SW
(5V/div)
I
INDUCTOR
(500mA/div)
V
IN
= 3V
V
OUT
= 10V
R
SET
= 10k
C
SS
= 0.01µF
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.
