EP5357LUI datasheet(12/17 Pages) ENPIRION

12 / 17 page

03409

September 12, 2012

Rev: D

EP5357LUI/EP5357HUI

www.enpirion.com

For applications with ENABLE control, tie LLM

to ENABLE; enable device after VIN ramp up is

complete and disable device before VIN ramp

down begins. For devices with ENABLE and

LLM tied to VIN, contact Enpirion Applications

engineering for specific recommendations

Increased output filter capacitance and/or

increased bulk capacitance at the load will

decrease the magnitude of the LLM ripple.

Refer to the section on output filter capacitance

for maximum values of output filter capacitance

and the Soft-Start section for maximum bulk

capacitance at the load.

NOTE:

For

proper

LLM

operation

the

EP5357xUI requires a minimum difference

between VIN and VOUT of 600mV.

If this

condition is not met, the device cannot be

assured proper LLM operation.

NOTE:

Automatic LLM/PWM is not available

when using the external resistor divider option

for VOUT programming.

Soft Start

Internal soft start circuits limit in-rush current

when the device starts up from a power down

condition or when the “ENABLE” pin is

asserted “high”. Digital control circuitry limits

the VOUT ramp rate to levels that are safe for

the Power MOSFETS and the integrated

inductor.

The EP5357HUI has a soft-start slew rate that

is twice that of the EP5357LUI.

When the EP5357LUI is configured in external

resistor divider mode, the device has a fixed

VOUT ramp time. Therefore, the ramp rate will

vary with the output voltage setting. Output

voltage ramp time is given in the Electrical

Characteristics Table.

Excess bulk capacitance on the output of the

device can cause an over-current condition at

startup. The maximum total capacitance on

the output, including the output filter capacitor

and bulk and decoupling capacitance, at the

load, is given as:

EP5357LUI:

COUT_TOTAL_MAX = COUT_Filter + COUT_BULK = 200uF

EP5357HUI:

COUT_TOTAL_MAX = COUT_Filter + COUT_BULK = 100uF

EP5357LUI in external divider mode:

COUT_TOTAL_MAX = 2.25x10-4/VOUT Farads

The nominal value for COUT is 10uF. See the

applications section for more details.

Over Current/Short Circuit Protection

The current limit function is achieved by

sensing the current flowing through a sense P-

MOSFET which is compared to a reference

current. When this level is exceeded the P-

FET is turned off and the N-FET is turned on,

pulling VOUT low. This condition is maintained

for approximately 0.5mS and then a normal

soft start is initiated.

If the over current

condition still persists, this cycle will repeat.

Under Voltage Lockout

During initial power up an under voltage

lockout circuit will hold-off the switching

circuitry until the input voltage reaches a

sufficient level to insure proper operation. If

the voltage drops below the UVLO threshold

the lockout circuitry will again disable the

switching. Hysteresis is included to prevent

chattering between states.

Enable

The ENABLE pin provides a means to shut

down

the

converter

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.

NOTE:

The ENABLE pin must not be left

floating.

Thermal Shutdown

When excessive power is dissipated in the

chip, the junction temperature rises. Once the

junction temperature exceeds the thermal

shutdown temperature the thermal shutdown

circuit turns off the converter output voltage

thus allowing the device to cool. When the

junction temperature decreases by 15C°, the

device will go through the normal startup

process.

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