MP1470B – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1470B Rev. 1.0

www.MonolithicPower.com

11

8/25/2015

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OPERATION

The

MP1470B

is

a

high-frequency,

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.

Internal Regulator

The 5V internal regulator powers most of the

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.

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 1MΩ 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

EN input current to less than 100μA.

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.

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.

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,

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.

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.