MCP8021 datasheet(41/66 Pages) MICROCHIP

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DS20006265D-page 41

MCP8021/2

5.0

APPLICATION INFORMATION

5.1

Component Calculations

5.1.1

CHARGE PUMP CAPACITORS

FIGURE 5-1: Charge Pump.

Let:

•IOUT = 20 mA

•fCP = 75 kHz (charge/discharge in one cycle)

• 50% duty cycle

•VDDH = 5.5V (worst case)

•RDSON = 7.5 (RPMOS), 3.5 (RNMOS)

• V12P = 2

 VDDH (ideal)

•CESR = 20 m (ceramic capacitors)

•VDROP = 100 mV (VOUT ripple)

•TCHG= TDCHG = 0.5  1/75 kHz = 6.67 µs

5.1.1.1

Flying Capacitor

The flying capacitor should be chosen to charge to a

minimum of 95% (3

) of VDDH within one half of a

switching cycle.

•3

 = TCHG

•

 = TCHG/3

• RC = TCHG/3

•C = TCHG/(R  3)

• C = 6.67 µs/([9

 + 5 + 0.02]  3)

• C = 158 nF

Choose a 180 nF capacitor.

5.1.1.2

Charge Pump Output Capacitor

Solve for the charge pump output capacitance,

connected between V12P and ground, that will supply

the 20 mA load for one switch cycle. The VBOOT pin on

the MCP8021/2 is the “V12P” pin referenced in the

calculations.

•C = IOUT  dt/dV

•C = IOUT  13.3 µs/(VDROP + IOUT  CESR)

•C = 20 mA

 13.3 µs/(0.1V + 20 mA  20 m)

•C

 2.65 µF

For stability reasons, the VBOOT LDO and VREG LDO

capacitors must be at least 4.7 µF, so choose: C

 4.7 µF.

5.1.1.3

Charging Path (Flying Capacitor

Across CAP1 and CAP2)

•VCAP = VDDH (1 – e-T/)

•VCAP = 5.5V (1 – e-[6.67 µs/([7.5 + 3.5 + 20 m] x 180 nF)])

VCAP = 5.31V available for transfer on the first cycle.

5.1.1.4

Transfer Path (Flying and Output

Capacitors)

• V12P = VDDH + VCAP – IOUT  dt/C

• V12P = 5.5V + 5.31V – (20 mA

 6.67 µs/180 nF)

• V12P = 10.066V

5.1.1.5

Calculate the Flying Capacitor

Voltage Drop in One Cycle While

Supplying 20 mA

•dV = IOUT  dt/C

•dV = 20 mA

 6.67 µs/180 nF

• dV = 0.741V @ 20 mA

The second and subsequent transfer cycles will have

a higher voltage available for transfer, since the

capacitor is not completely depleted with each cycle.

VCAP will then be VCAP – dV after the first transfer,

plus VDDH – (VCAP – dV) times the RC constant. This

repeats for each subsequent cycle, allowing a larger

charge pump capacitor to be used if the system will

tolerate several charge transfers before requiring full

output voltage and current.

Repeating Section 5.1.1.3 “Charging Path (Flying

Capacitor Across CAP1 and CAP2)” for the second

cycle (and subsequent by recalculating for each new

value of VCAP after each transfer):

•VCAP = (VCAP – dV) + (VDDH – (VCAP – dV)) (1 – e-T/t)

• VCAP = (5.31V – 0.741V) + (5.5V – (5.31V – 0.741V)) 

(1 – e-[6.67 µs/([7.5W + 3.5W + 20 mW]  180 nF)])

•VCAP = 4.567V + 0.934V  0.96535

VCAP = 5.468V available for transfer on second cycle.

5.1.1.6

Charge Pump Results

The maximum charge pump flying capacitor value is

202 nF to maintain a 95% voltage transfer ratio on the

first charge pump cycle. Larger capacitor values may

be used, but they will require more cycles to charge to

maximum voltage. The minimum required output

capacitor value is 2.65 µF to supply 20 mA for 13.3 µs

with a 100 mV drop. A larger output capacitor may be

used to cover losses due to capacitor tolerance over

temperature, capacitor dielectric and PCB losses.

These are approximate calculations. The actual

voltages may vary due to incomplete charging or dis-

charging of capacitors per cycle due to load changes.

The charge pump calculations assume the charge

pump is able to charge up the external boot cap within

a few cycles.

Transfer

Charge

VDDH

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