MCP8024 datasheet(19/46 Pages) MICROCHIP

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 2013 Microchip Technology Inc.

DS20005228A-page 19

MCP8024

The maximum inductor value for operation in

Discontinuous Conduction mode can be determined

by the following equation.

EQUATION 4-1:

LMAX SIMPLIFIED

Using the LMAX inductor value calculated using

Equation 4-1 will ensure Discontinuous Conduction

mode operation for output load currents below the crit-

ical current level, IO(CRIT). For example, with an output

voltage of +5V, a standard inductor value of 4.7

H will

ensure Discontinuous Conduction mode operation

with an input voltage of 6V, a switching frequency of

468 kHz, and a critical load current of 150 mA.

The output voltage is set by using a resistor divider

network. The resistor divider is connected between the

inductor output and ground. The divider common point

is connected to the FB pin which is then compared to

an internal 1.25V reference voltage.

The Buck regulator will set a Status bit and send a sta-

tus message to the host whenever the input switching

current exceeds two amperes peak (typical). The bit

will be cleared when the peak input switching current

drops back below the two ampere (typical) limit.

The Buck regulator will set a Status bit and send a sta-

tus message to the host whenever the output voltage

drops below 90% of the rated output voltage. The bit

will be cleared when the output voltage returns to 94%

of rated value.

If the Buck regulator output voltage falls below 80% of

rated output voltage, the system will shutdown with a

“Brown-out Error”. This will notify the Host of a power

failure and subsequent loss of configuration.

The Voltage Supervisor is designed to shutdown the

buck regulator when VDD rises above OVLOSTOP.

When shutting down the buck regulator is not desir-

able, the user should add a voltage suppression

device to the VDD input in order to prevent VDD from

rising above OVLOSTOP.

The Voltage Supervisor is also designed to shutdown

the buck regulator when VDD falls below UVLOSTOP.

4.1.4

CHARGE PUMP

An unregulated charge pump is utilized to boost the

input to the +12V LDO during low-input conditions.

When the input bias to the device (VDD) drops below

CPSTART, the charge pump is activated. When activated,

2 x VDD is presented to the input of the +12V LDO,

which maintains a minimum +10V at its output.

The typical charge pump flying capacitor is a 0.1 µF to

1.0 µF ceramic capacitor.

4.1.5

SUPERVISOR

The bias generator incorporates a voltage supervisor

and a temperature supervisor.

4.1.5.1

Voltage Supervisor

The voltage supervisor protects the device, external

power MOSFETs, and the external microcontroller

from damage due to overvoltage or undervoltage of

the input supply, VDD.

In the event of an undervoltage condition, VDD <

+5.5V, the motor drivers are switched off. The bias

generator, communication port, and the remainder of

the motor control unit remain active. The failure state

is flagged on the DE2 pin with a status message. In

extreme overvoltage conditions, VDD > +32V, all func-

tions are turned off.

4.1.5.2

Temperature Supervisor

An integrated temperature sensor self protects the

device circuitry. If the temperature rises above the

overtemperature shutdown threshold, all functions are

turned off. Active operation resumes when the

temperature has cooled down below a set hysteresis

value and the fault has been cleared by toggling CE.

It is desirable to signal the microcontroller with a warn-

ing message before the overtemperature threshold is

reached. The microcontroller should take appropriate

actions to reduce the temperature rise. The method to

signal the microcontroller is through the DE2 pin.

4.2

MOTOR CONTROL UNIT

The motor control unit is comprised of the following:

• External Drive for a 3-Phase Bridge with NMOS/

NMOS MOSFET pairs

• Three Motor Current Sense Amplifiers

• Motor Overcurrent Comparator

4.2.1

MOTOR CURRENT SENSE

CIRCUITRY

The internal motor current sense circuitry consists of

an operational amplifier and comparator. The amplifier

output is presented to the inverting comparator input

and as an output to the microcontroller. The non-

inverting comparator input is connected to an internally

programmable 8-bit DAC. A selectable motor current

limit threshold may be set with a SET_ILIMIT message

from the host to the MCP8024 via the DE2

communications link. The 8-bit DAC is powered by the

5V supply. The DAC output voltage range is 0.991V to

4.503V. The DAC has a bit value of (4.503V - 0.991V) /

(2^8 - 1) = 13.77 mV/bit. A DAC input of 00H yields a

DAC output voltage of 0.991V. The default power-up

DAC value is 40H (1.872V).

The DAC uses a 100

kHz filter. Input code to output voltage delay is

MAX

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