MP2269 - 30V, 1A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MP2269 Rev. 1.0

www.MonolithicPower.com

12

10/11/2016

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© 2016 MPS. All Rights Reserved.

LDO1

LDO2

VIN

BIAS

VCC

EN1

EN2

D1

Figure 3: VCC Regulator

Bootstrap Charging

The

bootstrap

capacitor

is

charged

and

regulated to about 5V by the dedicated internal

bootstrap regulator. When the voltage between

the BST and SW nodes is lower than its

regulation, a PMOS connected from VCC to

BST is turned on. The charging current path is

from VCC to BST to SW. When the HS-FET is

on, VIN is about equal to SW, so the bootstrap

capacitor cannot be charged.

At higher duty cycle operation conditions, the

time period available to the bootstrap charging

is less, so the bootstrap capacitor may not be

charged sufficiently. If the internal circuit does

not have sufficient voltage or time to charge the

bootstrap capacitor, extra external circuitry can

be used to ensure that the bootstrap voltage is

in the normal operation region.

Low-Dropout Operation

To improve dropout, the MP2269 is designed to

operate at almost 100% duty cycle as long as

the BST to SW voltage is greater than 2.5V.

When the voltage from BST to SW drops below

2.5V, the HS-FET is turned off using an under-

voltage lockout (UVLO) circuit that allows the

LS-FET to conduct and refresh the charge on

the BST capacitor. In DCM or PSM, the LS-FET

is forced on to refresh the BST voltage.

Since the supply current sourced from the BST

capacitor is low, the HS-FET can remain on for

more switching cycles than are required to

refresh the capacitor, making the effective duty

cycle of the switching regulator high.

The effective duty cycle during the dropout of

the regulator is mainly influenced by the voltage

drops across the power MOSFET, inductor

resistance, low-side diode, and printed circuit

board resistance.

Enable (EN) Control

EN is a digital control pin that turns the

regulator on and off. EN can be enabled by an

external logic H/L signal. When EN is pulled

below the threshold voltage, the chip is put into

the lowest shutdown current mode. Forcing EN

above the EN threshold voltage turns on the

part.

For

the

programmable

VIN

under-voltage

lockout (UVLO), with a high-enough VIN, the

chip can be enabled and disabled by EN (see

Figure

4).

This

circuit

can

generate

a

programmable VIN UVLO and hysteresis.

EN

VIN

Figure 4: Enable Divider Circuit

Frequency Programmable

The

MP2269

oscillating

frequency

is

FREQ to ground.

The

approximate

value

of

be

calculated with Equation (1):

freq

s

86500

R

(k )

6.5

f (kHz)

 



(1)