EM5845VT datasheet(11/13 Pages) EXCELLIANCE

11 / 13 page

2016/02/24

A.2

EM5845

Functional Description

Power Good Indicator

The power good output is an open-drain output

which requires a pull-up resistor. When the output

voltage is 10% below the nominal voltage, PG is

pulled low. It is held low until the output voltage

returns to the nominal voltage. PG is held low

during soft-start and activated about 1ms after

output voltage reaches its regulation.

Over Temperature Protection

Over thermal protection limits the total power

dissipation in the device, when the junction

temperature exceeds 150℃, an internal thermal

sensor shut down the device and allowing the IC to

cooling down. The device will turn on again after

the junction temperature cools by 25℃.

Switching Frequency Variation

The switching frequency varies with load current as

a results of the power losses in the MOSFETs and

the inductor. For a traditional PWM constant

frequency converter, as the load increases the duty

cycle also increases slightly to compensate for total

losses in the integrated MOSFETs and DCR losses in

the inductor. A constant on time control converter

must also compensate this kind of losses. The on

time is essentially fixed for a given Vo and VIN

combination, to compensate the losses the

off-time will tend to reduce slightly by constant on

time loop controlled as load increases. For this

reason, the switching frequency increases slightly

with increasing load.

Application Information

Input Capacitor Selection

The first objective in selecting input capacitors is to

reduce the ripple voltage amplitude seen at the

input of the converter. Ceramic capacitors placed

right at the input of the regulator reduce ripple

voltage. These capacitors must be placed close to

the converter input pins to be effective. High

quality ceramic input capacitor, such as X5R or X7R

and greater than 10uF are recommended for the

input capacitor due to more stable temperature

characteristic.

The input ripple voltage of converter is decided by

the value of input capacitance. Input ripple voltage

△

Vin can be calculated by the following equation.

Max







)

(

The RMS current rating of the input capacitor

should be larger than the following equation.

)

(

RMS









Inductor Selection

For a given input and output voltage, the inductor

value and switching frequency determine the

inductor ripple current. The ripple current increases

with

higher

VIN

and

decreases with

high

inductance, the lower the ripple current reduces

the core losses in the inductor, and output ripple

voltage. A good starting point for selecting the

ripple current is

)

(

MAX







, so the inductor

value should be chosen according to the following

equation:

)

(

)

(

)

(

MAX











When selecting an inductor, make sure that it is

capable of supporting the peak output current

without saturating. Inductor saturation will result in

a sudden reduction in inductance that may cause

the unstable operation.

Output Voltage Setting Resistor Selection

The device with an external adjustable output

voltage, the output voltage is programmed by an

external resistor divider connected from VO to VFB

and then to AGND, as shown in the typical

application schematic. The programmed output

voltage is below,

)

(







Where VFB is equal to the internal reference voltage

0.6V, R1 is the resistance from Vo to FB, R2 is from

VFB to AGND, and increases the quiescent current

by R1 and R2, so larger resistance is desirable,

select the R2 no larger than 200kΩis preferable.

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