Rev 0.95 – March 2005

www.enpirion.com

EN5310

6

Theory of Operation

Synchronous Buck Converter

The EN5310 is a synchronous, pin programmable

power supply with integrated power MOSFET

switches and inductor. The nominal input voltage

range is 2.5-5.0V. The output can be set to common

voltages by connecting appropriate combinations of 3

voltage selection pins to ground. If different voltage

levels are required, provision is also made to allow

external programming. The feedback control loop is

voltage-mode and the part uses a low-noise PWM

topology. Up to 1A of output current can be drawn

from this converter. The 5MHz operating frequency

enables the use of small-size output capacitors.

The power supply also has protection features such as:

• Programmable over-current protection (to

protect the IC from excessive load current)

• Thermal shutdown (to protect the converter

from getting too hot)

• Over-voltage protection that stops the PWM

switching and turns on the lower N-MOSFET

at 120% of the programmed output voltage in

order to protect the load from an OV

condition.

• Under-voltage lockout circuit to disable the

converter output when the input voltage is less

than approximately 2.2V

Additional features include:

• Soft-start circuit, limiting the in-rush current

when the converter is powered up.

• Power good circuit indicating whether the

output voltage is within 90%-120% of the

programmed voltage.

Output Voltage Programming

The EN5310 output voltage is programmed using one

of two methods. Common output voltages are

achieved by tying one or more of the three Voltage

Select pins (VS0, VS1 & VS2) to ground (see Table

1). If all three are left floating, the output voltage and

over voltage thresholds are determined by the voltages

presented at the XFB and XOV pins respectively.

These voltages should be set by way of resistor

going to XFB and XOV.

Use an appropriate resistor ratio such that the desired

output voltage across the resistor pair causes the XFB

pin to be 0.8V at the nominal set-point. It is

recommended that the resistor values used should be

in the 20K-40K range. Contact Enpirion Applications

Support for more specific information. If Over-

Voltage protection is required, the output of a second

divider to XOV should be set such that 0.96V is

present at the desired trip point. By design, if both

resistor dividers are the same, the OV trip-point will

be 20% above the nominal output voltage.

XFB

22µF

10µF

15 nF

VOUT

POK

PGND

AGND

SS

PVIN

AVIN

XOV

V

OUT

V

IN

Table 1: Output Voltage Select Table

VS2*

VS1*

VS0*

Output Voltage

0

0

0

3.3V

0

0

1

2.5V

0

1

0

1.8V

0

1

1

1.5V

1

0

0

1.25V

1

0

1

1.2V

1

1

0

0.8V**

1

1

1

User Selectable

** 0.8V ref only, not guaranteed performance

* The VS0, VS1 and VS2 pins are defaulted to a ‘1’

with an internal pull-up resistor. Only connect

these pins to AGND if a ‘0’ is required. If a ‘1’ is

required, then leave the pin floating.

Capacitor Selection

The EN5310 needs about 10-20uF of input

capacitance. Low-cost, low-ESR ceramic capacitors

can be used as input capacitors for this converter and

it is strongly recommended that they be rated X5R or

X7R. In some applications, lower value capacitors are

needed in parallel with the larger, lossy capacitors in

order to provide high frequency decoupling.