MIC2145 Micrel Semiconductor, MIC2145 Datasheet - Page 7

• Manufacturer Part Number: MIC2145
• Description: High Efficiency 2.5W Boost Converter
• Manufacturer: Micrel Semiconductor

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 A 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
“Output Voltage”
May 2003
MIC2145
“Application Information”
Figure 2. Typical Regulator Waveforms
(100mV/div)
(500mA/div)
AC Couple
I
(5V/div)
INDUCTOR
V
V
OUT
SW
section).
for component selection and
Time 20 s
OUT
experiences a typical
DD
. This can be
FF
(See
7
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
Current Limit vs. R
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.
Thermal Shutdown
Built-in thermal protection circuitry turns off the power MOSFET
when the junction temperature exceeds about 150 C.
Figure 3. Typical Soft Start Waveforms
(500mA/div)
I
INDUCTOR
(2V/div)
(5V/div)
(5V/div)
V
V
V
OUT
SW
EN
SET
SET
”
.
at lower V
“Typical Characteristics: Peak
Time 200 s
DD
voltages. For resistor
C
SS
V
R
OUT
SET
= 0.01 F
V
IN
= 10V
= 10k
= 3V
MIC2145
Micrel