MP1470B – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
OPERATION
The
MP1470B
is
a
high-frequency,
Connecting EN directly to a voltage source
without a pull-up resistor requires limiting the
amplitude of the voltage source to ≤6V to
prevent damage to the Zener diode.
synchronous, rectified, step-down, switch-mode
converter with internal power MOSFETs. It
offers a very compact solution to achieve a 2A
continuous output current over a wide input
supply range with excellent load and line
regulation.
The MP1470B operates in a fixed frequency,
peak-current-control mode to regulate the
output voltage. An internal clock initiates the
PWM cycle to turn on the integrated high-side
power MOSFET. This MOSFET remains on
until its 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
the current in the power MOSFET does not
reach the COMP set current value within 92%
of one PWM period, the power MOSFET is
forced to turn off.
Figure 2: 6.5V Zener Diode
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) protects the chip
from operating at an insufficient supply voltage.
The MP1470B UVLO comparator monitors the
output voltage of the internal regulator (VCC).
The typical value of the UVLO rising threshold
is about 4.2V while its falling threshold is about
3.86V.
Internal Regulator
The 5V internal regulator powers most of the
internal circuits. This regulator takes VIN and
operates in the full VIN range.
Pre-Bias Start-Up
The MP1470B is designed for a monotonic
start-up into pre-biased loads. If the output is
pre-biased to a certain voltage during start-up,
the BST voltage is charged by VIN. If the BST
voltage exceeds its rising threshold voltage, the
internal soft-start voltage ramps up. When the
internal soft-start voltage exceeds the sensed
output voltage at FB, the part works normally.
Error Amplifier
The error amplifier compares the FB voltage
against the internal 0.8V reference (REF) and
outputs a current proportional to the difference
between the two. This output current charges or
discharges the internal compensation network
to form the COMP voltage, which is used to
control the power MOSFET current. The
optimized internal compensation network
minimizes the external component count and
simplifies the control loop design.
Internal Soft Start (SS)
A soft start prevents the converter output
voltage from overshooting during start-up.
When the chip starts up, the internal circuit
generates a soft-start voltage (SS) that ramps
up from 0V to 1.2V. When SS falls below the
internal reference (REF), SS overrides REF, so
the error amplifier uses SS as the reference.
When SS exceeds REF, the error amplifier
resumes using REF as its reference. The SS
time is set internally to 1ms.
Enable (EN)
EN is a digital control pin that turns the
regulator on and off. Drive EN high to turn on
the regulator; drive EN low to turn off the
regulator. An internal 1Mꢀ resistor from EN to
GND allows EN to float to shut down the chip.
EN is clamped internally using a 6.5V series
Zener diode (see Figure 2). Connecting the EN
input through a pull-up resistor to VIN limits the
EN input current to less than 100μA.
For example, with 12V connected to VIN,
RPULLUP ≥ (12V-6.5V) ÷ 100μA = 55kꢀ.
MP1470B Rev. 1.0
8/25/2015
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
11
MPS [ MONOLITHIC POWER SYSTEMS ]
