MCP1633 datasheet(16/31 Pages) MICROCHIP

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MCP1633

DS20006289A-page 16

 2020 Microchip Technology Inc.

4.4

Error Amplifier/Comparator

Current Limit Function

The error amplifier is of transconductance type (OTA)

with external compensation network connected

between the output (COMP) and the ground. The error

amplifier generates an error signal proportional to the

difference between the LED current sense feedback

voltage and VREF input voltage. The error amplifier

output is clamped by a precision 2.7V internal voltage

source. The output of the error amplifier is connected to

the inverting input of the high-speed comparator. As the

output load current demand increases, the error ampli-

fier output increases too, causing the inverting input pin

of the high-speed comparator to increase. Eventually,

the output of the error amplifier will hit the clamp, limit-

ing the input of the high-speed comparator to 2.7V

maximum. Even if the FB input continues to decrease,

calling for more current, the inverting input is limited to

2.7V. By limiting the inverting input to 2.7V, the current

sense (IP) input is limited to 2.7V minus DC offset value

(CSO), thus limiting the current that flows in the main

switch. Limiting the maximum peak current in the

switch prevents the destruction of the semiconductor

device and the saturation of the inductor during over-

loads. To simplify the design when CS signal is too

small so peak limit is far away, there is another feature

called I peak offset (CSO). This DC value is added over

current sense information to avoid output overloads.

Refer to Figure 4-2 and Figure 4-3 for a detailed

description. The error amplifier input includes only the

GND potential.

FIGURE 4-2:

Slope Compensation Signal.

FIGURE 4-3:

Slope Compensation

Circuit.

4.5

Reference Voltage Generator

The internal precision constant current generator and

an external resistor connected between the VREF pin

and GND form the reference voltage generator. Refer

to Figure 4-4 for details. Optionally, a capacitor (CSS)

can be connected in parallel with RVREF to activate the

soft start function that will minimize overshoots of the

output voltage during start-up. The equations in

Figure 4-4 calculate the value of the resistor (RVREF)

for a given reference voltage and the value of the soft

start capacitor (CSS) based on the necessary time to

reach 90% of the final value for VREF. An internal circuit

of the MCP1633 device will discharge the capacitor

during the shutdown period. This capacitor must be of

good quality, with low leakage currents, in order to

avoid any errors that can affect the reference voltage.

The reference voltage should not exceed 80% of the

bias input voltage (VIN pin) in order to avoid any errors

that affect the internal constant current generator. An

external low-noise, low-impedance source can be used

to overdrive the VREF pin in order to control the refer-

ence voltage. In this case, the resistor/capacitor group

connected to GND is not necessary, and the soft start

profile must be controlled by the external reference

voltage generator.

Amplitude (V)

Slope

Compensation

Generator

Time

Slope

DC Offset

CSO

Peak Limit

OSCILLATOR

VEXT

RAMP

6 K

To PWM

Comparator

RSENSE

RSLOPE

CFILTER

CSO

ROFFSET

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