MCP1650 datasheet(14/28 Pages) MICROCHIP

14 / 28 page

MCP1650/51/52/53

DS21876A-page 14

 2004 Microchip Technology Inc.

4.7

FB Pin

The output voltage is fed back through a resistor divider

to the FB pin. It is then compared to an internal 1.22V

reference. When the divided-down output is below the

internal reference, the internal oscillator is gated on

and the EXT pin pulses the external N-channel

MOSFET on and off to transfer energy from the source

to the load at 750 kHz. This will cause the output volt-

age to rise until it is above the 1.22V threshold, thereby

gating the internal oscillator off. Hysteresis is provided

within the comparator and is typically 12 mV. The rate

at which the oscillator is gated on and off is determined

by the input voltage, load current, hysteresis voltage

and inductance. The output ripple voltage will vary

depending on the input voltage, load current,

hysteresis voltage and inductance.

4.8

PWM Latch

The gated oscillator is self-latched to prevent double

and sporadic pulsing. The reset into the latch is asyn-

chronous and can terminate the pulse during the on-

time of the duty cycle. The reset can be accomplished

by the feedback voltage comparator or the current limit

comparator.

4.9

Peak Inductor Current

The external switch peak current is sensed on the CS

pin across an optional external current sense resistor.

If the CS pin falls more than 122 mV (typical) below

VIN, the current limit comparator is set and the pulse is

terminated. This prevents the current from getting too

high and damaging the N-channel MOSFET. In the

event of a short circuit, the switch current will be low

due to the current limit. However, there is a DC path

from the input through the inductor and external diode.

This is true for all boost-derived topologies and addi-

tional protection circuitry is necessary to prevent

catastrophic damage.

4.10

EXT Output Driver

The EXT output pin is designed to directly drive

external N-channel MOSFETs and is capable of

sourcing 400 mA (typical) and sinking 800 mA (typical)

for fast on and off transitions. The top side of the EXT

driver is connected directly to VIN, while the low side of

the driver is tied to GND, providing rail-to-rail drive

capability. Design flexibility is added by connecting an

external resistor in series with the N-channel MOSFET

to control the speed of the turn on and off. By slowing

the transition speed down, there will be less high-

frequency noise. Speeding the transition up produces

higher efficiency.

4.11

Low Battery Detect

The Low-Battery Detect (MCP1651 and MCP1653

only) feature can be used to determine when the LBI

input voltage has fallen below a predetermined

threshold.

The

low-battery

detect

comparator

continuously monitors the voltage on the LBI pin. When

the voltage on the LBI pin is above the 1.22V + 123 mV

hysteresis, the LBO pin will be high-impedance (open-

drain). When in the high-impedance state, the leakage

current into the LBO pin is typically less than 0.1 µA. As

the voltage on the LBI pin decreases and is lower than

the 1.22V typical threshold, the LBO pin will transition

to a low state and is capable of sinking up to 10 mA.

123 mV of hysteresis is provided to prevent chattering

of the LBO pin as a result of battery input impedance

and boost input current.

4.12

Power Good Output

The Power Good Output feature (MCP1652 and

MCP1653 only) monitors the divided-down voltage

feedback into the FB pin. When the output voltage falls

more than 15% (typical) below the regulated set point,

the power good (PG) output pin will transition from a

high-impedance state (open-drain) to a low state

capable of sinking 10 mA. If the output voltage rises

more than 15% (typical) above the regulated set point,

the PG output pin will transition from high to low.

4.13

Device Protection

4.13.1

OVERCURRENT LIMIT

The Current Sense (CS) input pin is used to sense the

peak input current of the boost converter. This can be

used to limit how high the peak inductor current can

reach. The current sense feature is optional and can be

bypassed by connecting the VIN input pin to the CS

input pin. Because of the path from input through the

boost inductor and boost diode to output, the boost

topology cannot support a short circuit without

additional circuitry. This is typical of all boost regulators.

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