EM5845VT datasheet(10/13 Pages) EXCELLIANCE

10 / 13 page

2016/02/24

A.2

EM5845

Functional Description

Theory of Operation

The EM5845 is a step-down dc-dc regulator that

uses pseudo-fixed frequency constant-on-time

control architecture with integrated high-side

switch and low-side synchronous rectifier. The high

switch frequency and tiny 3X3-10 pin DFN package

enables the user to optimize the solution for

minimum board space and efficiency.

The

buck

regulator

employs

pseudo-fixed

frequency constant-on time control. This control

method allows fast transient response than

traditional current-mode control scheme thereby

reducing the total component count due to

lowering the size of the output capacitance.

Constant On-Time Control

The PWM control scheme used by constant

on-time control, the high side switch on-time is

determined by internal pulse generator. The pulse

period is determined by VO and VIN information

from VIN and SW pin; the period is proportional to

output voltage and inversely proportional to input

voltage. The advantages of this control scheme are

such as fast transient response, without external

compensation and less output component count

than other control method.

Light Load Power Saving Mode

The EM5845 offers an automatic pulse-skipping

mode in order to provide excellent light load

efficiency. The regulator senses the zero current

point when the low-side gate is turned on and

prevents the negative current flow by turning off

the low-side gate signal. This saves power by

eliminating re-circulation of the inductor current.

When the low-side MOS is turned off, the regulator

will enter discontinuous mode that reducing the

switching

loss

due

switching

frequency

decreased. During this time, both power switches

are shut off, and the output capacitor supplies all

light load current. Because the output voltage dips

and recovers occasionally, the output voltage ripple

in this mode is larger than the ripple in the heavy

load operation.

Soft-Start and Soft-Stop Function

The EM5845 has built-in soft-start function that

ramps up the output voltage at constant slew rate

to avoid output overshooting at start-up. The

internal soft-start time is typically about 1ms.

The output soft-stop operation is active when the

EN from high to low. An internal discharge resistor

typically 100Ω is enabled in this condition to

discharge the output capacitor through SW pin.

Chip Enable

When the input voltage exceeds the under voltage

lockout (UVLO) threshold 2.4V typically, the

EM5845 will be enabled by pulling the EN pin high.

Leaving the EN pin floating or grounded will disable

the chip. There is an internal 1MΩ resistor from the

EN pin to ground.

Input UVLO Protection on VCC and VIN

The EM5845 has input under voltage lockout

protection (UVLO), if the input voltage exceeds the

UVLO on threshold 2.4V typically, and there is

typically a 500us delay for internal circuit waking up

then soft-start begins. The device will shut off if VCC

or VIN falls below UVLO level.

Current limit Protection

The EM5845 has a minimum 4.8A current limit

level for high side switch. If the internal high side

sensing current is above this level, the FET will stop

switching immediately and low-side FET will be

turned on until the current level drops below the

hysteresis window of current limit level.

Output Under-Voltage Protection

Output

under-voltage

protection

works

conjunction with the current limit protection. If the

FB voltage drops below 50% internal VREF, after a

delay of 7.5us, the converter will be latched off.

Under-voltage protection can be released by

toggling the EN/VIN or VCC.

Output Over-Voltage Protection

If the FB voltage above 130% internal VREF, after a

delay of 7.5us, the converter will be latched off.

Over-voltage protection can be released by toggling

the EN/VIN or VCC.

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