MP1494 – SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
The MP1494 is a high-frequency, synchronous,
rectified, step-down, switch-mode converter
with built-in power MOSFETs. It offers a very
compact solution that achieves a 2A continuous
output current with excellent load and line
regulation over a wide input supply range.
The MP1494 operates in a fixed-frequency,
peak-current–control mode to regulate the
output voltage. An internal clock initiates a
PWM cycle. The integrated high-side power
MOSFET turns on and remains on until the
current reaches the value set by the COMP
voltage. When the power switch is off, it
remains off until the next clock cycle starts. If, in
within 95% of one PWM period, the current in
the power MOSFET does not reach the value
set by the COMP value, the power MOSFET is
forced to turn off.
Internal Regulator
A 5V internal regulator powers most of the
internal circuitries. This regulator takes the V
IN
input and operates in the full V
IN
range. When
V
IN
is greater than 5.0V, the output of the
regulator is in full regulation. When V
IN
is lower
than 5.0V, the output decreases, and the part
requires a 0.1µF ceramic decoupling capacitor.
Error Amplifier
The error amplifier compares the FB pin voltage
against the internal 0.8V reference (REF) and
outputs the COMP voltage—COMP controls the
power MOSFET current. The optimized internal
compensation network minimizes the external
component count and simplifies the control loop
design.
Enable/SYNC control
EN/SYNC is a digital control pin that turns the
regulator on and off. Drive EN high to turn on
the regulator; drive it low to turn it off. An
internal 1MΩ resistor from EN/SYNC to GND
allows EN/SYNC to be floated to shut down the
chip.
The EN pin is clamped internally using a 6.7V
series-Zener-diode as shown in Figure 2.
Connecting the EN input pin through a pullup
resistor to the voltage on the V
IN
pin limits the
EN input current to less than 100µA.
For example, with 12V connected to Vin,
R
PULLUP
≥
(12V – 6.5V) ÷ 100µA = 55kΩ.
Connecting the EN pin is directly to a voltage
source without any pullup resistor requires
limiting the amplitude of the voltage source to
≤6V
to prevent damage to the Zener diode.
Figure 2: 6.5V Zener Diode Connection
For external clock synchronization, connect a
clock with a frequency range between 200kHz
and 2MHz 2ms after the output voltage is set:
The internal clock rising edge will synchronize
with the external clock rising edge. Select an
external clock signal with a pulse width less
than 1.7μs.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) protects the chip
from operating at insufficient supply voltage.
The MP1494 UVLO comparator monitors the
output voltage of the internal regulator, VCC.
The UVLO rising threshold is about 3.9V while
its falling threshold is 3.25V.
Internal Soft-Start
The soft-start prevents the converter output
voltage from overshooting during startup. When
the chip starts, the internal circuitry generates a
soft-start voltage (SS) that ramps up from 0V to
1.2V. When SS is lower than REF, the error
amplifier uses SS as the reference. When SS is
higher than REF, the error amplifier uses REF
as the reference. The SS time is internally set
to 1.5ms.
Over-Current-Protection and Hiccup
The MP1494 has a cycle-by-cycle over-current
limit when the inductor current peak value
exceeds the set current limit threshold.
Meanwhile, the output voltage drops until FB is
below the Under-Voltage (UV) threshold—
typically 50% below the reference. Once UV is
triggered, the MP1494 enters hiccup mode to
periodically restart the part. This protection
mode is especially useful when the output is
dead-shorted to ground. The average short
MP1494 Rev. 1.04
12/26/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
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