Rev 0.95 – March 2005

www.enpirion.com

EN5310

7

The EN5310 has been optimized for use with about

20µF of ceramic output capacitance. It is strongly

recommended that these be low-cost, low-ESR,

ceramic capacitors rated X5R or X7R. (See the

Enpirion application note on ripple comparison for

optimum selection of number and value of these

capacitors based on ripple requirements.) In order to

eliminate high-frequency switching spikes on the

output ripple, usually a low-value, low-ESR ceramic

capacitor is used in parallel with the larger capacitors

right at the load.

Enable Operation

The ENABLE pin provides a means to shut down the

power FET switching or enable normal operation. A

logic low will disable the converter and cause it to

shut down. A logic high will enable the converter into

normal operation.

Soft-Start Operation

The SS pin in conjunction with a small capacitor

between this pin and AGND provides the soft start

function to limit the in-rush current during start-up.

During start-up of the converter the reference voltage

to the error amplifier is gradually increased to its final

level by an internal current source of typically 10uA.

The whole soft-start procedure is designed to take 1ms

- 3ms with a 15-30nF soft start capacitor, but can be

programmed by capacitor selection using the

following equation:

POK Operation

The POK signal is an open drain signal from the

converter indicating the output voltage is within the

specified range. The POK signal will be a logic high

when the output voltage is within 90% - 120% of the

programmed output voltage. If the output voltage goes

outside of this range, the POK signal will be a logic

low until the output voltage has returned to within this

range. In the event of an over-voltage condition the

POK signal will go low and will remain in this

condition until the output voltage has dropped to 95%

of the programmed output voltage before returning to

the high state (see also Over Voltage Protection)

Over-Current Protection

The cycle-by-cycle current limit function is achieved

by sensing the current flowing through the sense P-

MOSFET and a signal generated by a differential

amplifier with a preset over-current threshold. During

a particular cycle, if the over-current threshold is

exceeded, the power P-MOSFET is turned off and the

power N-MOSFET is turned on to protect the P-

MOSFET. If the over-current condition is removed,

the over-current protection circuit will enable the

PWM operation. If the over-current condition persists,

the converter will eventually go through a full soft-

start cycle. This circuit is designed to provide high

noise immunity.

It is possible to adjust the over-current set point by

connecting a resistor between ROCP and GND

(increase the trip point) or PVIN (decrease the trip

point). The voltage at the ROCP pin is designed to be

0.8V. (see application note for details)

Over-Voltage Protection

When the output voltage exceeds 120% of the

programmed output voltage, the PWM operation

stops, the lower N-MOSFET is turned on and the

POK signal goes low. When the output voltage drops

below 95% of the programmed output voltage, normal

PWM operation resumes and POK returns to its high

state.

Thermal Overload Protection

Thermal shutdown will disable operation once the

Junction temperature exceeds approximately 160ºC.

Once the junction temperature drops by approx 25ºC,

the converter will re-start with a normal soft-start.

Low Input Voltage Operation

Circuitry is provided to ensure that when the input

voltage is below the specified voltage range, the

operation of the converter is controlled and

predictable. Circuits for hysteresis, input de-glitch and

output leading edge blanking are included to ensure

high noise immunity and prevent false tripping.

Compensation

The EN5310 is internally compensated through the

use of a type 3 compensation network and is

optimized for use with about 20µF of output

capacitance and will provide excellent loop bandwidth

and transient performance for most applications. (See