TC1235 datasheet(4/10 Pages) MICROCHIP

4 / 10 page

Inverting Dual (–VIN, –2VIN)

Charge Pump Voltage Converters

with Shutdown

TC1235

TC1236

TC1237

TC1235/6/7-1 3/24/00

DS21371A

P2LOSS (2, 3, 4) = IOUT2

2 x R

OUT2

where ROUT2 = [ 1 / [fOSC(C2) ] + 8RSWITCH2 +

4ESRC2 + ESRCOUT2 ]

Equation 1b.

The internal switch resistance for the first stage (i.e.

RSWITCH1) is approximately 3Ω and the switch resistance for

the second stage (i.e. RSWITCH2) is approximately 7Ω.

The losses in the circuit due to factor (4) above are also

shown in Equation 2a for stage 1 and Equation 2b for stage

2. The output voltage ripple for stage 1 is given by Equation

3a and the output voltage ripple for stage 2 is given by

Equation 3b.

PLOSS1 (4) = [ (0.5)(C1)(VIN

2 – V

OUT1

2 ) + (0.5)

(COUT1) (VRIPPLE1

2 - 2V

OUT1 VRIPPLE1) ] x fOSC

Equation 2a.

PLOSS2 (4) = [ (0.5) (C2) (VIN

2 – V

OUT2

2 ) + (0.5)

(COUT2) (VRIPPLE2

2 - 2V

OUT2 VRIPPLE2) ] x fOSC

Equation 2b.

VRIPPLE1 = [ IOUT1 / (fOSC) (COUT1) ] + 2 (IOUT1)

(ESRCOUT1)

Equation 3a.

VRIPPLE2 = [ IOUT2 / (fOSC) (COUT2) ] + 2 (IOUT2)

(ESRCOUT2)

Equation 3b.

Capacitor Selection

In order to maintain the lowest output resistance and

output ripple voltage, it is recommended that low ESR

capacitors be used. Additionally, larger values of C1 and C2

will lower the output resistance and larger values of COUT1

and COUT2 will reduce output ripple. (See Equations 1a, 1b,

3a, and 3b). NOTE: For proper charge pump operation,

C1 and COUT1 must have a voltage rating greater than or

equal to VIN, while C2 and COUT2 must have a voltage

rating greater than or equal to 2VIN.

Table 1a shows various values of C1 and the corre-

sponding output resistance values for VIN=5V @ +25°C for

stage 1 and Table 1b shows various values of C2 and the

corresponding output resistance values for VIN=5V @ +25°C

for stage 2. It assumes a 0.1

Ω ESRC1, a 0.1Ω ESRC2, a 3Ω

RSWITCH1, and a 7Ω RSWITCH2.

Table 2a shows the output voltage ripple for various

values of COUT1 and Table 2b shows the output voltage

ripple for various values of COUT2 (again assuming VIN = 5V

@ +25

oC). The VRIPPLE1 values assume a 3mA output load

current for stage 1 and a 0.1

Ω ESRCOUT1. The VRIPPLE2

values assume a 200

µA output load current for stage 2 and

a 0.1

Ω ESRCOUT1.

Table 1a. Output Resistance vs. C1 (ESR = 0.1

Ω). For Stage 1

C1 (

µF) TC1235 R

OUT (

Ω) TC1236 R

OUT (

Ω) TC1237 R

OUT (

Ω)

0.47

202

108

3.3

Table 1b. Output Resistance vs. C2 (ESR = 0.1

Ω). For Stage 2

C2 (

µF) TC1235 ROUT (Ω) TC1236 ROUT (Ω) TC1237 ROUT (Ω)

0.1

890

342

137

0.47

239

117

140

Table 2a. Output Voltage Ripple vs. COUT1 (ESR = 0.1Ω) For Stage 1

(IOUT1 = 3mA)

COUT1

TC1235 VRIPPLE1

TC1236 VRIPPLE1

TC1237 VRIPPLE1

(

µF)

(mV)

0.47

533

183

251

3.3

Table 2b. Output Voltage Ripple vs. COUT2 (ESR = 0.1Ω) For Stage 2

(IOUT2 = 200

µA)

COUT2

TC1235 VRIPPLE2

TC1236 VRIPPLE2

TC1237 VRIPPLE2

(

µF)

(mV)

0.1

167

0.47

3.4

5.8

1.6

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