MCP1650 datasheet(17/28 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21876A-page 17

MCP1650/51/52/53

5.2

Design Considerations

When developing switching power converter circuits,

there are numerous things to consider and the

MCP1650/51/52/53 family is no exception. The gated

oscillator architecture does provide a simple control

approach so that stabilizing the regulator output is an

easier task than that of a fixed-frequency regulator.

The MCP1650/51/52/53 controller utilizes an external

switch and diode allowing for a very wide range of

conversion (high voltage gain and/or high current gain).

There are practical, as well as power-conversion,

topology limitations. The MCP1650/51/52/53 gated

oscillator hysteretic mode converter has similar

limitations, as do fixed-frequency boost converters.

5.2.1

DESIGN EXAMPLE

Setting the output voltage:

By adjusting the external resistor divider, the output

voltage of the boost converter can be set to the desired

value. Due to the RC delay caused by the resistor

divider and the device input capacitance, resistor

values greater than 100 k

Ω are not recommended. The

feedback voltage is typically 1.22V.

For this example:

5.2.1.1

Calculations

For gated oscillator hysteretic designs, the switching

frequency is not constant and will gate several pulses

to raise the output voltage. Once the upper hysteresis

threshold is reached, the gated pulses stop and the

output will coast down at a rate determined by the out-

put capacitor and the load. Using the gated oscillator

switching frequency and duty cycle, it is possible to

determine what the maximum boost ratio is for

continuous inductor current operation.

This relationship assumes that the output load current

is significant and the boost converter is operating in

Continuous Inductor Current mode. If the load is very

light or a small boost inductance is used, higher boost

ratio’s can be achieved.

Calculate at minimum VIN:

The ideal maximum output voltage is 14V. The actual

measured result will be less due to the forward voltage

drop in the boost diode, as well as other circuit losses.

For applications where the input voltage is above and

below 3.8V, another point must be checked to deter-

mine the maximum boost ratio. At 3.8V, the duty cycle

changes from 80% to 56% to minimize the peak current

in the inductor.

For this case, VOUTMAX = 8.63V less than the required

12V output specified. The size of the inductor has to

decrease in order to operate the boost regulator in

Discontinuous Inductor Current mode.

Input Voltage = 2.8V to 4.2V

Output Voltage = 12V

Output Current = 100 mA

Oscillator Frequency = 750 kHz

Duty cycle = 80% for VIN < 3.8V

Duty cycle = 56% for VIN > 3.8V

RBOT =10 kΩ

VOUT = 12V

VFB = 1.22V

RTOP = 88.4 kΩ

90.9 K

Ω was selected as the closest standard value.

RTOP

RBOT

VOUT

VFB

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



 1

–





Where:

RTOP = Top Resistor Value

RBOT = Bottom Resistor Value

POUT

VOUT IOUT

Where:

POUT = 12V X 100 mA

POUT = 1.2 Watts

POUT Efficiency

()

⁄

Where:

PIN = 1.2W/80%

PIN = 1.5 Watts

(80% is a good efficiency estimate)

VOUT

–

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



 V

VOUTMAX

10.8

–

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



 2.8

VOUTMAX

10.56

–

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



 3.8

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