MP1474 – SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
The MP1474 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 MP1474 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,
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 off.
Internal Regulator
A 5V internal regulator powers most of the
internal circuitries. This regulator takes V
IN
and
operates in the full V
IN
range. When V
IN
exceeds 5.0V, the output of the regulator is in
full regulation. When V
IN
is less 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
to the internal 0.807V reference (V
REF
) and
outputs a current proportional to the difference
between the two. This output current then
charges
or
discharges
the
internal
compensation network to form the COMP
voltage, which controls the power MOSFET
current. The optimized internal compensation
network minimizes the external component
counts 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 6.5V
series-Zener-diode as shown in Figure 2.
Connecting the EN input pin through a pullup
resistor to the voltage on the IN pin limits the
EN input current to less than 100µA.
For example, with 12V connected to IN, 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)
The MP1474 has under-voltage lock-out
protection (UVLO). When the VCC voltage
exceeds the UVLO rising threshold voltage, the
MP1474 powers up. It shuts off when the VCC
voltage drops below the UVLO falling threshold
voltage. This is non-latch protection.
The MP1474 is disabled when the input voltage
falls below 3.25V. If an application requires a
higher under-voltage lockout (UVLO) threshold,
use the EN pin as shown in Figure 3 to adjust
the input voltage UVLO by using two external
resistors. For best results, set the UVLO falling
threshold (VSTOP) above 4.5V using the
enable resistors. Set the rising threshold
(VSTART) to provide enough hysteresis to
allow for any input supply variations.
MP1474 Rev. 1.0
9/26/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
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