MCP8024 datasheet(29/46 Pages) MICROCHIP

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DS20005228A-page 29

MCP8024

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

•VDD = 6V (worst case)

• RDSON = 7.5  (RPMOS), 3.5  (RNMOS)

•Vout = 2 x VDD (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 VDD within one half of a

switching cycle.

3 *

= Tchg

 = Tchg/3

RC = Tchg/3

C = Tchg/(R * 3)

C = 6.67 µs/([7.5

 + 3.5 + 0.02] * 3)

C = 202 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 12VLDO pin

on the MCP8024 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

5.1.1.3

Charging Path (Flying Capacitor

across CAP1 and CAP2)

VCAP = VDD (1 - e -T/t)

VCAP = 6V (1 - e -[6.67 µs / ([7.5 + 3.5 + 20 m] * 180 nF)])

VCAP = 5.79V available for transfer

5.1.1.4

Transfer Path (Flying and Output

Capacitors)

V12P = VDD + VCAP - IOUT * dt / C

V12P = 6V + 5.79V - (20 mA * 6.67 µs / 180 nF)

V12P = 11.049V

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 VDD -

(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 for the second cycle (and

subsequent by re-calculating for each new value of

VCAP after each transfer):

VCAP = (VCAP - dV) + (VDD - (VCAP - dV)) (1 - e -T/t)

VCAP = (5.79V - 0.741V) + (6V - (5.79V - 0.741V) *

(1 - e-[6.67 μs/([7.5Ω + 3.5Ω + 20 mΩ] * 180 nF)])

VCAP = 5.049V + 0.951V * 0.96535

VCAP = 5.967V 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 volt-

ages may vary due to incomplete charging or discharg-

ing 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

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