MCP1643 datasheet(12/28 Pages) MICROCHIP

12 / 28 page

MCP1643

DS20005208B-page 12

 2013-2023 Microchip Technology Inc. and its subsidiaries

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The MCP1643 synchronous boost regulator operates

at 0.5V input. The maximum output voltage range is

limited by overvoltage protection at 5.0V. LED current

stays in regulation while VIN  VOUT minus a

300 mV – 400 mV headroom. The power efficiency

conversion is high when driving LED currents up to

hundreds of mA. Output current capability is limited by

the 1.6A typical peak input current limit. Typical charac-

terization curves in this data sheet are presented to dis-

play the typical output current capability.

5.2

LED Brightness Control

5.2.1

ADJUSTABLE CONSTANT

CURRENT CALCULATIONS

To calculate the resistor values for the MCP1643’s LED

current, use Equation 5-1, where RSET is connected to

VFB and GND. The reference voltage (VFB) is 120 mV.

EQUATION 5-1:

EXAMPLE 1:

EXAMPLE 2:

Power dissipated on the RSET resistor is very low and

equal with VFB*ILED. For 100 mA LED current, the

power dissipated on sense resistor is only 12 mW, and

the efficiency of the conversion is high.

Equation 5-1 applies for one or even two LEDs in

series connection. The Typical Applications graphic

shows the maximum and minimum limits for RSET over

the input voltage range that ensures current regulation

for a white LED.

5.2.2

PWM DIMMING

LED’s brightness can also be controlled by setting a

maximum current allowed for LED (using Equation 5-1)

and lowering it in small steps with a variable duty cycle

PWM signal applied to the EN pin. The maximum fre-

quency for dimming is limited by the soft start, which

varies with the LED current. By varying the duty cycle

of the signal applied on the EN pin (from 0 to 100%),

the LED current is changing linearly (see Figure 2-8).

5.3

Input Capacitor Selection

The boost input current is smoothed by the boost

inductor, reducing the amount of filtering necessary at

the input. Some capacitance is recommended to

provide decoupling from the source. Low ESR X5R or

X7R are well suited, since they have a low temperature

coefficient and small size. For most applications,

4.7 µF of capacitance is sufficient at the input. For high-

power applications that have high-source impedance

or long leads, connecting the battery to 10 µF capaci-

tance is recommended. Additional input capacitance

can be added to provide a stable input voltage.

5.4

Output Capacitor Selection

The output capacitor helps provide a stable output

voltage and smooth load current during sudden load

transients, as is the PWM dimming. Ceramic capacitors

are well suited for this application (X5R and X7R). The

range of the output capacitor vary from 4.7 µF (in case

of light loads and static applications) up to 20 µF (for

hundreds of milliamp LED currents and PWM dimming

applications).

5.5

Connecting More LEDs to Output

White LEDs have a typical 2.7V to 3.2V forward voltage

(VF), which depends on the power dissipated according

to its VF/IF characteristic. Because MCP1643 allows up

to 5.0V maximum to output, two white LEDs in series

connection are not possible.

Two or more white LEDs can be connected in parallel

to output, as shown in Figure 6-1. Current sensing is

necessary only for one LED. Each LED of the string is

passed by the calculated current according to

Equation 5-1. A protection circuit formed by a Zener

and general purpose diodes will protect the rest of

LEDs, if the LED in the sense loop fails.

Two red, green or yellow LEDs can be connected in

series to the output of MCP1643 (see application

example on Figure 6-2). Red LEDs have a typical VF

between 1.8V and 2.2V (it depends on the real color),

yellow LEDs have the VF between 2.1V and 2.2V, while

for green options, consider values from 2.0V to 2.4V.

VFB = 120 mV

ILED =25 mA

RSET =4.8Ωwith a standard value of 4.7Ω

ILED is 25.53 mA)

VFB =120 mV

ILED =100 mA

RSET =1.2Ω

ILED

VFB

RSET

------------

MCP1643 Datasheet(PDF) 12 Page - Microchip Technology

MCP1643 Datasheet(HTML) 12 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL