MCP1601_13 datasheet(13/24 Pages) MICROCHIP

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DS21762B-page 13

MCP1601

5.2.1.1

Input

For all BUCK-derived topologies, the input current is

pulled from the source in pulses, placing some burden

on the input capacitor. For most applications, a 10 µF

ceramic capacitor connected to the MCP1601 input is

recommended to filter the current pulses. Less capaci-

tance can be used for applications that have low source

impedance. The ripple current ratings for ceramic

capacitors are typically very high due to their low loss

characteristics. Lower-cost electrolytic capacitors can

be used, but ripple current ratings should not be

exceeded.

5.2.1.2

Output

For BUCK-derived topologies, the output capacitor fil-

ters the continuous AC inductor ripple current while

operating in the PWM mode. Typical inductor AC ripple

current for the MCP1601 is 120 mA peak-to-peak with

a 3.6V input, 10 µH inductor for a 1.8V output applica-

tion. Using an output capacitor with 0.3

 of ESR, the

output ripple will be approximately 36 mV.

The recommended range for the output capacitor is

from 10 µF (±20%) to 47 µF (±20%). Larger value

capacitors can be used, but require evaluation of the

control system stability.

EQUATION

The above equation assumes that the output capaci-

tance is large enough so that the ripple voltage (as a

result of charging and discharging the capacitor) is

negligible and can be used for applications that use

electrolytic capacitors with esr > 0.3



When using a 10 µF ceramic X5R dielectric capacitor,

the output ripple voltage is typically less than 10 mV.

5.2.2

BUCK INDUCTOR

There are many suppliers and choices for selecting the

BUCK inductor. The application, physical size require-

ments (height vs. area), current rating, resistance,

mounting method, temperature range, minimum induc-

tance and cost all need to be considered in making the

best choice.

When choosing an inductor for the MCP1601 Synchro-

nous

BUCK,

there

are

two

primary

electrical

specifications to consider.

Current rating of the inductor.

Resistance of the inductor.

When selecting a BUCK inductor, many suppliers

specify a maximum peak current.

The maximum peak inductor current is equal to the

maximum DC output current plus 1/2 the peak-to-peak

AC ripple current in the inductor. The AC ripple current

in the inductor can be calculated using the following

relationship.

EQUATION

Solving for

I

EQUATION

Example:

The

approximate

“on”

time

equal

the

Duty Cycle (VOUT / VIN) x 1/FSW.

Many suppliers of inductors rate the maximum RMS

(Root Mean Square) current. The BUCK inductor RMS

current is dependent on the output current, inductance,

input voltage, output voltage and switching frequency.

For the MCP1601, the inductor RMS current over the

2.7V to 5.5V input range, 0.9V to 5V output voltage

range is no more than 15% higher than the average DC

output current for the minimum recommended induc-

tance of 10 µH ±20%. When selecting an inductor that

has a maximum RMS current rating, use a simple

approximation that the RMS current is 1.2 times the

maximum output current.

Example:

IOUT(MAX) = 300 mA, the inductor should have an RMS

rating > 360 mA (1.2 x IOUT(MAX)).

Ripple

LRipple

OUTesr



VIN =3.6V

VOUT =1.8V

FSW =750 kHz

IOUT(MAX) =300 mA

TON =

(1.8V/3.6V) x 1/(750 kHz)

TON =

667 ns

VL =

3.6V - 1.8V = 1.8V

I

L =

(1.8V/10 µH) x 667 ns

I

L =120 mA

IL(PEAK) =IOUTMAX + 1/2 IL

IL(PEAK) =

300 mA + (120 mA) / 2

IL(PEAK) =360 mA



I



 t



Where:

t is equal to the “on” time of the P-Channel

switch and,

VL = the voltage across the inductor

(VIN - VOUT)

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