MCP1633 datasheet(17/31 Pages) MICROCHIP

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 2020 Microchip Technology Inc.

DS20006289A-page 17

MCP1633

FIGURE 4-4:

Reference Voltage Generator.

4.6

Internal Oscillator

The MCP1633 PWM controller provides programmable

switching

frequency

with

external

resistor

connected between the FSW pin and GND. To set a

desired frequency, the resistor value can be calculated

with the Equation 1 below or using the graph in

Figure 2-20. A small capacitor in parallel with RFSW is

recommended.

EQUATION 1:

RESISTOR VALUE.

FIGURE 4-5:

Switching Frequency Set.

The internal oscillator can be synchronized by an exter-

nal clock pulse to EN/Sync pin. It is recommended that

the frequency of the external synchronization pulse is

within ±20% of the internal oscillator frequency

programmed by the FSW resistor. If the external

synchronization clock is lost, then the internal oscillator

takes control of the switching rate based on the FSW

resistor to maintain similar operating conditions. FSW

pin cannot be left floating.

4.7

Current Sense Amplifier

The internal current sense amplifier measures LED

current based on the differential voltage drop between

the ISP and ISN inputs over a common mode range of

0V to 5V. Inputs and output of the current sense

amplifier are directly connected to pins (ISP, ISN, IOUT)

to set the gain of the amplifier by external resistors

providing high flexibility in designing applications. The

output of current sense amplifier should be connected

to FB pin to close the loop and regulate the output

current. An optional low-pass filter can be used to filter

the effects of output current ripple and switching

spikes. Figure 4-6 shows an example of a filter.

SS/VREF

VIN

μA

VREF

RVREF

CSS

Time(s)

VREF

0.9*VREF

Amplitude(V)

RVREF 



VREF V



A

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

=CSS F



2.3 RVREF k





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

RFSW

2E 05

–

SW2



0.0234 Fsw



0.026

–

FSW

VIN

μA

Oscillator

RFSW

CFSW

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