User`s guide

MCP1630 Boost Mode LED Driver Demo Board User’s Guide

The CCP output is also connected to a simple ramp generator that is reset at the begin-

ning of each MCP1630 clock cycle. The ramp generator is composed of transistor Q2,

resistors R2, R3 and capacitor C6. It provides the reference signal to the MCP1630

comparator. The MCP1630 comparator compares this ramp reference signal to the

error amplifier output to generate a PWM signal.

A resistor voltage divider (R4, R5 and R6) and filter is used to set the reference voltage

for the constant current control and is driven by the GP5 pin of the MCU. Jumper J4 is

used to select two different reference voltages to drive the LED load at different current

levels. With J4 removed, resistors R4 and R5 provide a 206 mV reference voltage. This

setting provides 350 mA output current. With J4 installed, resistors R4, R5 and R6

provide a 412 mV reference voltage. (700 mA output)

R8 and C7 form an integrator circuit in the negative feedback path, providing high loop

gain at DC. This simple compensation network is sufficient for a constant current LED

driver. Places have been provided on the PCB for additional compensation

components, if required.

R12 and R13 form a voltage divider that is used to monitor the output voltage of the

boost circuit. The output of the voltage divider is connected to pin GP4 of the MCU and

monitored with the ADC.

L1, Q1, D2, C12, and C13 form the basic voltage boost circuit. C13 is not populated on

the PCB. The value of C12 has been selected to keep the LED current ripple less than

20% at the rated load conditions.

2.5 FIRMWARE DESCRIPTION

Appendix C shows the firmware flowchart. Different parts of the firmware operation are

described below.

2.5.1 Clock Generation

At a minimum, the PIC12F683 code must initialize the CCP module in PWM mode to

produce a clock signal for the MCP1630. The CCP is configured to produce 500 kHz

with a 20% duty cycle. The clock duty cycle limits the maximum on time of the power

MOSFET to 80% duty. I/O pins GP5 is driven high to enable the voltage reference for

the MCP1630 control loop. After these tasks are completed, the MCP1630 can begin

the current regulation function and the MCU can execute other tasks.

2.5.2 Open Circuit Detection

One potential problem that can occur with a constant current application is an open-cir-

cuit load condition. In this case, the power supply will try to raise the output voltage to

provide the required current. A boost converter circuit can continue to raise the output

voltage until component damage occurs. The software monitors the output voltage to

the MCP1630 using an ADC channel and disables the power supply when a maximum

output voltage of 40V is reached.

Once the output reference voltage reaches the allowable maximum limit, the GP2 pin

is driven high by writing a 100% duty cycle value to the CCP control registers. This high

signal effectively resets the MCP1630. The reference to the MCP1630 is also disabled

by clearing pin GP5. After the over-voltage event, the software provides a delay before

restarting the input clock to the MCP1630 PWM generator and turning on the reference

voltage. The delay reduces stress on the power supply components.