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ADM00435

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
MCP1643 SYNCHRONOUS BOOST LED
CONSTANT CURRENT REGULATOR
EVALUATION BOARD USER’S GUIDE
2013 Microchip Technology Inc. DS20005196A-page 13
Chapter 2. Installation and Operation
2.1 INTRODUCTION
The MCP1643 has been developed for applications that require driving an LED from a
low-voltage source. The Microchip low-voltage technology allows the device to start up
from a low 0.65V input voltage without output overshoot. At the same time, the device
is capable of achieving high efficiency by integrating the low resistance N-Channel
Boost switch and the synchronous P-Channel switch and by using a low voltage refer-
ence (0.120V) to reduce the power dissipation on the sensing resistor. Another import-
ant feature is that the device integrates the compensation and protection circuitry, so
that the final solution will require a minimum number of additional components.
When disabled using the EN switch, the true disconnect option removes the normal
boost topology path from input to output.
FIGURE 2-1: Synchronous Boost 25, 50, 75, 100 mA Constant Current Application.
2.1.1 MCP1643 Evaluation Board Features
The MCP1643 Synchronous Boost LED Constant Current Regulator Evaluation Board
has the following features:
• It can be powered by one-cell Alkaline, NiCd, or NiMH batteries, or by external
power supply
• Input voltage range, V
IN
: 0.5V to 2.5V, with V
IN
V
OUT
, after start-up
• Start-up voltage: 0.65V
• Fixed output current: 25 mA, 50 mA, 75 mA or 100 mA, selected using a mini-dip
switch on board
• PWM Switching Frequency: 1 MHz
• Enable state selectable using mini-dip switch on board
• 1.6A Peak Input Current Limit
• Overtemperature Protection (if the die temperature exceeds 150°C, 25°C hysteresis)
• Mechanical battery reverse polarity protection
V
IN
GND
V
FB
C
OUT
2X10µF
C
IN
10 µF
L
1
4.7 µH
SW
LED
4.7
EN
V
OUT
+
-
ALKALINE
MCP1643
R
SET0
4.7
R
SET1
2.4
R
SET2
EN
REN
1M
Note: For V
IN
< V
OUT
, I
LED
remains in regulation up to V
IN
= V
LED
@ LED typical
V
F
and I
F.
MCP1643 Synchronous Boost LED Constant Current Regulator Evaluation Board User’s Guide
DS20005196A-page 14 2013 Microchip Technology Inc.
2.2 GETTING STARTED
The MCP1643 Synchronous Boost LED Constant Current Regulator Evaluation Board
is fully assembled and tested to evaluate and demonstrate the MCP1643 product. This
board requires the use of an external power supply or an AA battery.
2.2.1 Power Input and Output Connection
2.2.1.1 POWERING THE MCP1643 SYNCHRONOUS BOOST LED CONSTANT
CURRENT REGULATOR EVALUATION BOARD
Soldered test points are available for input voltage connections. The maximum input
voltage should not exceed 2.5V. The output current will not remain in regulation for
input voltages that are greater than, or equal to, the forward voltage of the LED. White
LEDs have V
F
typical between 2.5 and 3.5V, depending on the LED drive current.
The MCP1643 Synchronous Boost LED Constant Current Regulator Evaluation Board
was designed to be used in the process of validating the device. The package selected
for the MCP1643 Synchronous Boost LED Constant Current Regulator Evaluation
Board is the MSOP-8.
SW1 is the Enable switch, which gives the state of the converter, ON or OFF. A sol-
dered test point that can be used for PWM dimming is also available for the EN pin. The
second switch is used to modify the value of the sense resistor, in order to modify the
LED current and achieve analog dimming (for the position of the switches and output
current, see Table 2-1).
2.2.1.2 BOARD POWER UP PROCEDURE
1. Connect the input supply as shown in Figure 2-2 or connect an AA battery in the
battery holder on the bottom of the board.
2. Use SW1 to change the state of the converter from ON to OFF.
3. Change the value of the output current using SW2. There are four available
currents: 25 mA, 50 mA,75 mA and 100 mA.
Additional test points are available to visualize different signals (SW, output current,
FB) or to modify the output current by dimming, using the EN pin. Note: Due to the
holder reverse polarity protection, the battery can only be inserted in the correct
position.
FIGURE 2-2: MCP1643 Synchronous Boost LED Constant Current Regulator
Evaluation Board Setup.
+
_
Power Supply
Installation and Operation
2013 Microchip Technology Inc. DS20005196A-page 15
2.2.1.3 ADJUSTABLE LED CURRENT SETTING BY RECALCULATING THE
SENSE RESISTOR
The Sense Resistor is used to modify the value of the output current. The value for the
sense resistor can be calculated using the following equation:
R
2
=V
FB
/I
LED
Where: V
FB
= 0.120V
2.2.1.4 ADJUSTABLE LED CURRENT SETTING BY USING THE EN PIN (PWM
DIMMING)
The MCP1643 allows PWM dimming by turning the LED ON or OFF with a variable
duty cycle PWM signal applied to the EN pin. The maximum frequency for dimming is
limited by the internal soft-start of 240 µs typical. By varying the duty cycle of the PWM
signal applied on EN input, the LED current is changing linearly and the light intensity
changes as well.
A 0Ω R1 resistor is used for bench testing. By removing R1, the soldered test points
can be used to either insert a multimeter to measure the LED current, or create a loop
and visualize the current through the LED by using an oscilloscope current probe.
TABLE 2-1: LED CURRENT SELECTION
Switch Position LED Current
25 mA
50 mA
75 mA
100 mA
ON
2
1
ON
2
1
ON
2
1
ON
2
1
Note: If the sense resistor is recalculated, the switch used for current selection will
not be used, as it will add a resistor in parallel with the calculated
component. When recalculating the sense resistor, check the Maximum
Limits for I
LED
in the Regulation graph available in the data-sheet.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22

ADM00435

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Manufacturer:
Microchip Technology
Description:
Power Management IC Development Tools MCP1643 0.5W LED Driver Demo Board
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