TC1142 datasheet(6/12 Pages) MICROCHIP

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TC1142

DS21360B-page 6

 2002 Microchip Technology Inc.

3.1

Output Voltage and Ripple

For a -2x boost:

In this case, the output voltage is given by:

EQUATION 3-1:

Here, f is the clock frequency and RS is the total ON

resistance of the switches connecting C2 to GND and

VOUT in phase 2 of the charge pump operating cycle

with the equivalent series resistance (ESR) of C2.

The output ripple voltage is given by:

EQUATION 3-2:

Here, ESR is the equivalent series resistance of COUT.

In this case, the TC1142 is held in phase 2 until the

output voltage drops below VR. When this occurs, the

TC1142 reverts to phase 1 for a half period of the clock,

during which C2 is charged from C1. At the end of this

half-period, C2 is reconnected to COUT to boost the

output voltage. During the phase 1 time period, the

output voltage will drop below VR before it is boosted

back, so the minimum output voltage is approximated

by:

EQUATION 3-3:

The output ripple voltage is given approximately by:

EQUATION 3-4:

For values of VIN higher than |VR/2| by several hundred

mV, the effect on ripple of the ESR of COUT can be

neglected compared to the “overdrive” effect of VIN.

Here, it can be seen that VRIPPLE increases with

increasing VIN, but can be minimized by choosing small

C1 and C2 values and a large COUT value.

3.2

Capacitor Selection

To maintain low output impedance and ripple, it is

recommended that capacitors with low equivalent

series resistance (ESR) be used. Additionally, larger

values of the output capacitor and smaller values of the

flying capacitors will reduce output ripple. For a

capacitor value of 4.7

µF for COUT, and values of

0.47

µF for C1 and C2, the typical output impedance of

the TC1142 in regulation is 0.5

Ω. For the capacitor ESR

not to have a noticeable effect on output impedance, it

should not be larger than 1/2fCOUT. This also makes its

effect on ripple voltage negligible. For VIN = 3.2V and

VR = -5V, the output ripple voltage is less than 70

mVPP. Table 3-1 summarizes output ripple versus

capacitor size for an input voltage of 3.2V and a

regulated output voltage of -5V.

Surface mount ceramic capacitors are preferred for

their small size, low cost and low ESR. Low ESR

tantalum capacitors also are acceptable. See Table 3-2

for a list of suggested capacitor suppliers.

TABLE 3-1:

VOLTAGE RIPPLE VS. C1/C2

FLYING CAPACITORS AND

OUTPUT CAPACITOR COUT

ESR = 0.1

Ω, IOUT = 20mA

a.) For unregulated operation when VIN ≤ |

VOUT = -|2VIN| + IOROUT

RSC2

(C2 + COUT)

)

(

where ROUT =

VRIPPLE = IORRIPPLE

ESR C2

2f(C2 + COUT) 2fCOUT (C2 + COUT)

where RRIPPLE =

b.) For regulated operation when VIN >

| |.

VOUTMIN = -|VR| + IOROUT

where ROUT =

ESR C2

2fCOUT

(C2 + COUT)

VRIPPLE =

(2VIN – |VR| + ESR IOC2

)

()

)

( +

where N =

(C2 + COUT)

C1, C2

(

µF)

COUT

(

µF)

VIN

(V)

VOUT

(V)

VRIPPLE

(mV)

0.1

4.7

3.2

-5

14.6

0.22

4.7

3.2

-5

31.4

0.33

4.7

3.2

-5

46.1

0.47

4.7

3.2

-5

63.9

0.68

4.7

3.2

-5

88.7

1.0

4.7

3.2

-5

123.2

0.1

3.2

-5

7.0

0.22

3.2

-5

15.1

0.33

3.2

-5

22.4

0.47

3.2

-5

31.5

0.68

3.2

-5

44.7

1.0

3.2

-5

63.8

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