MCP1631 datasheet(19/34 Pages) MICROCHIP

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DS22063B-page 19

MCP1631/HV/MCP1631V/VHV

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The MCP1631/MCP1631V can be used to develop

intelligent power management solutions, typical

applications include a multi-chemistry battery charger

used to charge Li-Ion, NiMH or NiCd batteries and

constant current LED drivers.

5.2

Battery Charger Design Overview

The design approach for developing high current

switching battery chargers using the MCP1631 is

described in this section. Depending on input voltage

range, there are two versions of the device that can be

used to accommodate a very wide range of input

voltages.

For a regulated input voltage range of 5V, the

MCP1631/MCP1631V device is used, for this input

voltage application (regulated ac-dc converter or USB

input), the MCP1631/MCP1631V is powered directly

from the 5V dc input.

For input voltages to +16V steady state with +18V

transients, the MCP1631HV/MCP1631VHV, or high

voltage option can be used. The high voltage devices

integrate a low dropout (LDO) linear regulator with a set

output voltage of +3.3V or +5.0V that internally powers

the MCP1631HV/MCP1631VHV and is also capable of

providing 250 mA of bias current for the attached

microcontroller and other circuitry. MCP1631HV/

MCP1631VHV internal power dissipation must be

considered when loading the internal LDO regulator.

For higher input voltages the MCP1631/MCP1631V

can be biased from an external regulated +3.0V to

+5.5V supply.

5.3

Programmable Single Ended

Primary Inductive (SEPIC) Current

Source

The MCP1631/MCP1631V family integrates features

that are necessary to develop programmable current

sources. The SEPIC converter is commonly used in

battery charger applications. The primary or input

inductor is used to filter input current and minimize the

switching noise at the converter input. The primary to

secondary capacitive isolation blocks any dc path from

input to output making the SEPIC safer than Buck or

other non-isolated topologies. The SEPIC rectifier

blocks the reverse path preventing battery leakage, in

other topologies an additional diode for blocking is

necessary

adding

additional

components

and

efficiency loss.

The input or primary inductor and output or secondary

inductor are typically constructed from a single

magnetic device with two windings, this is commonly

referred to as a coupled inductor. Using coupled

inductors has significant advantages in addition to the

size and cost benefits of a single core with multiple

windings.

5.4

Mixed Signal Design

For intelligent battery charger design, a microcontroller

is used to generate the proper charge profile, charge

termination, safety timers and battery charger features.

When using the MCP1631/MCP1631V for Li-Ion

battery charger applications, the microcontroller is also

used to generate the constant voltage regulation phase

of the charge cycle. This is accomplished by using the

external reference feature of the MCP1631/MCP1631V

as a programmable current source. The microcontroller

is used to vary the VREF input of the MCP1631/

MCP1631V. The charge current into the battery is

regulated by the MCP1631/MCP1631V, the level that it

is regulated to is set by the programmability of the

microcontroller.

The internal MCP1631/MCP1631V analog compo-

nents are used to regulate the microcontroller

programmed current. The secondary or battery current

is sensed using amplifier A2, the output of A2 is feed

into the input of the error amplifier A1, the output of A1

sets the peak switch current of the SEPIC converter, it

increases or decreases the battery current to match its

(A1) inputs. By increasing the VREF or non-inverting

input of A1, the battery current is increased.

5.5

Safety Features

The MCP1631/MCP1631V integrates a high-speed

comparator used to protect the charger and battery

from being exposed to high voltages if the battery is

removed or opens. Comparator C2 is used to sense the

SEPIC output voltage. If the divided down output

voltage becomes higher than the 1.2V internal

MCP1631/MCP1631V reference, the VEXT PWM

output is terminated within 50 ns preventing the build

up of voltage on the SEPIC output.

Peak switch current is limited by the MCP1631/

MCP1631V comparator C1 and error amplifier A1

output voltage clamp. For the MCP1631, the error

amplifier output is clamped at 2.7V. The A1 output is

divided down by 1/3 and compared with CS (current

sense) input. The VEXT output is turned off if the CS

input reaches a level of 1/3 of 2.7V or 0.9V in 12 ns,

preventing the external switch current from becoming

high enough to damage the SEPIC power train.

Internal overtemperature protection limits the device

junction temperature to 150°C preventing catastrophic

failure for overtemperature conditions. Once the

temperature decreases 10°C, the device will resume

normal operation.

Safety timers are typically used to limit the amount of

energy into a faulted battery or pack. This is

accomplished using the microcontroller and MCP1631/

MCP1631V shutdown feature.

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