MCP1650 datasheet(18/28 Pages) MICROCHIP

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MCP1650/51/52/53

DS21876A-page 18

 2004 Microchip Technology Inc.

determine

the

maximum

inductance

for

Discontinuous Operating mode, multiply the energy

going into the inductor every switching cycle by the

number of cycles per second (switching frequency).

This number must be greater than the maximum input

power.

The equation for the energy flowing into the inductor is

given below. The input power to the system is equal to

energy times time.

The inductor peak current is calculated using the

equation below:

Using a typical inductance of 3.3 µH, the peak current

in the inductor is calculated below:

At 3.8V and below, the converter can boost to 14V

while operating in the Continuous mode.

For this example, a 3.3 µH inductor is too large, a

2.2 µH inductor is selected.

As the inductance is lowered, the peak current drawn

from the input at all loads is increased. The best choice

of inductance for high boost ratios is the maximum

inductance

value

necessary

while

maintaining

discontinuous operation.

For lower boost-ratio applications (3.3V to 5.0V), a

3.3 µH inductor or larger is recommended. In these

cases, the inductor operates in Continuous Current

mode.

5.2.2

MOSFET SELECTION

There are a couple of key consideration’s when

selecting the proper MOSFET for the boost design. A

low

RDSON logic-level N-channel MOSFET is

recommended.

5.2.2.1

MOSFET Selection Process.

Voltage Rating - The MOSFET drain-to-source

voltage must be rated for a minimum of VOUT +

VFD of the external boost diode. For example, in

the 12V output converter, a MOSFET drain-to-

source voltage rating of 12V + 0.5V is

necessary. Typically, a 20V part can be used for

12V outputs.

Logic-Level RDSON - The MOSFET carries

significant current during the boost cycle on

time. During this time, the peak current in the

MOSFET can get quite high. In this example, a

SOT-23 MOSFET was used with the following

ratings:

Selecting MOSFETs with lower RDSON is not always

better or more efficient. Lower RDSON typically results

in higher total gate charge and input capacitance, slow-

ing the transition time of the MOSFET and resulting in

increased switching losses.

5.2.3

DIODE SELECTION

The external boost diode also switches on and off at the

switching frequency and requires very fast turn-on and

turn-off times. For most applications, Schottky diodes

are recommended. The voltage rating of the Schottky

diode must be rated for maximum boost output voltage.

For example, 12V output boost converter, the diode

should be rated for 12V plus margin. A 20V or 30V

Schottky diode is recommended for a 12V output appli-

cation. Schottky diodes also have low forward-drop

characteristics, another desired feature for switching

power supply applications.

FSW = 750 kHz

TON =(1/FSW * Duty Cycle)

IPK (2.8V) = 905 mA

Energy (2.8V) = 1.35 µ-Joules

Power (2.8V) = 1.01 Watts

IPK (3.8V) = 860 mA

Energy at 3.8V = 1.22 µ-Joules

Power = 0.914 Watts

FSW = 750 kHz

TON =(1/FSW * Duty Cycle)

IPK (2.8V) = 1.36A

Energy (2.8V) = 2.02 µ-Joules

Power (2.8V) = 1.52 Watts

IPK(3.8V) = 1.29A

Energy at 3.8V = 1.83 µ-Joules

Power = 1.4 Watts

Energy

--- LIPK

IPK

VIN

-------- TON

IRLM2502 N-channel MOSFET

VBDS = 20V (Drain Source Breakdown

Voltage)

RDSON = 50 milli-ohms (VGS = 2.5V)

RDSON = 35 milli-ohms (VGS = 5.0V)

QG = Total Gate Charge = 8 nC

VGS = 0.6V to 1.2V (Gate Source Threshold

Voltage)

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