MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP1570 Rev. 1.5

www.MonolithicPower.com

5

1/31/2006

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TM

OPERATION

FUNCTIONAL DESCRIPTION

The MP1570 is a synchronous rectified,

current-mode, step-down regulator. It regulates

input voltages from 4.75V to 23V down to an

output voltage as low as 1.230V, and supplies

up to 3A of load current.

The MP1570 uses current-mode control to

regulate the output voltage. The output voltage

is measured at FB through a resistive voltage

divider and amplified through the internal

transconductance error amplifier. The voltage at

COMP pin is compared to the switch current

measured internally to control the output

voltage.

The

converter

uses

internal

N-Channel

MOSFET switches to step-down the input

voltage to the regulated output voltage. Since

the high side MOSFET requires a gate voltage

greater than the input voltage, a boost capacitor

connected between SW and BS is needed to

drive the high side gate. The boost capacitor is

charged from the internal 5V rail when SW is low.

When the MP1570 FB pin exceeds 20% of the

nominal regulation voltage of 1.230V, the over

voltage comparator is tripped and latched; the

COMP pin and the SS pin are discharged to

GND, forcing the high-side switch off. Latch

cannot be cleared unless the EN or IN pin is

reset.

Following discharge, the MP1570 actively

recycles the energy stored in the output

capacitor. Initially the low-side synchronous

rectifier turns on. Once the internal, negative

900mA current limit is reached, the low-side

switch turns off, forcing inductor current to flow

through the high-side switch body diode. The

inductor current is recycled back into the input

as an energy saving feature. This cycle

continues until the output voltage is discharged

below 10% of the initial regulation voltage

(0.123V at FB), at which time the low-side

switch turns off.

APPLICATIONS INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to FB

pin. The voltage divider divides the output

voltage down to the feedback voltage by the

ratio:

2

R

1

R

2

R

V

V

OUT

FB

+

=

Thus the output voltage is:

2

R

2

R

1

R

23

.

1

+

×

=

the output voltage.

A typical value for R2 can be as high as 100kΩ,

but a typical value is 10kΩ. Using that value, R1

is determined by:

)

k

)(

23

.

1

V

(

18

.

8

1

R

OUT

Ω

−

×

=

For example, for a 3.3V output voltage, R2 is

10kΩ, and R1 is 17kΩ.

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will result in less ripple current that will

result in lower output ripple voltage. However,

the larger value inductor will have a larger

physical size, higher series resistance, and/or

lower saturation current. A good rule for

determining the inductance to use is to allow

the peak-to-peak ripple current in the inductor

to be approximately 30% of the maximum

switch current limit. Also, make sure that the