MCP8024 datasheet(18/46 Pages) MICROCHIP

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MCP8024

DS20005228A-page 18

 2013 Microchip Technology Inc.

4.0

DETAILED DESCRIPTION

4.1

BIAS GENERATOR

The internal bias generator controls three voltage rails.

Two fixed-output low-dropout linear regulators, an

adjustable buck switch-mode power converter, and an

unregulated charge pump are controlled through the

bias generator. In addition, the bias generator per-

forms supervisory functions.

4.1.1

+12V LOW-DROPOUT LINEAR

REGULATOR (LDO)

The +12V rail is used for bias of the 3-phase power

MOSFET bridge.

The regulator is capable of supplying 20mA of external

load current. The regulator has a minimum overcurrent

limit of 30 mA.

The low-dropout regulators require an output capacitor

connected from VOUT to GND to stabilize the internal

control loop. A minimum of 4.7

F ceramic output

capacitance is required for the 12V LDO.

4.1.2

+5V LOW-DROPOUT LINEAR

REGULATOR (LDO)

The +5V LDO is used for bias of an external microcon-

troller, the internal current sense amplifier and the gate

control logic.

The +5V LDO is capable of supplying 20mA of exter-

nal load current. The regulator has a minimum over-

current limit of 30 mA. If additional external current is

required, the buck switch-mode power converter

should be utilized.

A minimum of 4.7

F ceramic output capacitance is

required for the 5V LDO.

4.1.3

BUCK SWITCH-MODE POWER

CONVERTER (SMPS)

The SMPS is a high-efficiency, fixed-frequency, step-

down DC-DC converter. The SMPS provides all the

active functions for local DC-DC conversion with fast

transient response and accurate regulation.

During normal operation of the buck power stage, Q1

is repeatedly switched on and off with the on and off

times governed by the control circuit. This switching

action causes a train of pulses at the LX node which

are filtered by the L/C output filter to produce a DC

output voltage, VO. Figure 4-1 depicts the functional

block diagram of the SMPS.

FIGURE 4-1:

SMPS Functional Block

Diagram.

The SMPS is designed to operate in Discontinuous

Conduction Mode (DCM) with Voltage mode control

and current limit protection. The SMPS is capable of

supplying 5V, 150mA to an external load at a fixed

switching frequency of 460 kHz with an input voltage

of 6V. The output of the SMPS is power limited. There-

fore, for a programmed output voltage of 3V, the

SMPS will be capable of supplying 250mA to an exter-

nal load. An external diode is required between the LX

pin and ground. The diode will be required to handle

the inductor current when the switch is off. The diode

is external to the device to reduce substrate currents

and power dissipation caused by the switcher. The

external diode carries the current during the switch off

time, eliminating the current path back through the

device.

At light loads the SMPS enters Pulse Frequency

Modulation (PFM), improving efficiency at the expense

of higher output voltage ripple. The PFM circuitry

provides a means to disable the SMPS as well. If the

SMPS is not utilized in the application, connecting the

feedback pin (FB) to an external 2.5V-to-5.0V supply

will force the SMPS to a shutdown state.

OUTPUT

CONTROL

LOGIC

VIN

CURRENT_REF

VDD-12V

BANDGAP

REFERENCE

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