MCP1632-BAESLASHMS datasheet(15/34 Pages) MICROCHIP

15 / 34 page

 2013 Microchip Technology Inc.

DS20005254A-page 15

MCP1632

FIGURE 4-3:

Slope Compensation Signal

(CS) Pin.

4.7

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 MCP1632 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

reference 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.

FIGURE 4-4:

Reference Voltage

Generator.

4.8

Internal Oscillator

The

MCP1632

PWM

controller

provides

two

(orderable) switching frequency options: 300 kHz and

600 kHz.

4.9

Undervoltage Lockout (UVLO)

When the input voltage (VIN) is less than the UVLO

threshold, the VEXT is held in low state. This will ensure

that, if the voltage is not adequate to power the

MCP1632 device, the main power supply switch will be

held in off state. In order to prevent oscillations when

the input voltage is near the UVLO threshold, the UVLO

circuit offers 100 mV (typical) hysteresis. Typically, the

MCP1632 device will not start until the input voltage at

VIN is between 2.8V and 2.9V (typical).

4.10

Overtemperature Protection

To protect the VEXT output if shorted to VIN or GND, the

VEXT output of the MCP1632 device will be

high-impedance if the junction temperature is above

the thermal shutdown threshold. An internal 10 k



pull-down resistor is connected from VEXT to ground to

provide some pull-down during overtemperature

conditions. The protection is set to 150°C (typical), with

a hysteresis of 20°C.

Slope V



0.9 V



SLOPE

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



LOW



0.32 V



SLOPE

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



HIGH



1.22 V



SLOPE

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



Amplitude (V)

Slope

(V)

Time

DCLOW

DCHIGH

VREF





REF



A

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



2.3

VREF







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

VREF

VIN

50 μA

VREF

RVREF

CSS

Time(s)

VREF

0.9*VREF

Amplitude (V)

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