TC1142 Просмотр технического описания (PDF) - Microchip Technology
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TC1142 Datasheet PDF : 12 Pages
| |||
TC1142
3.1 Output Voltage and Ripple
For a -2x boost:
| | a.) For unregulated operation when VIN ≤
VR
2
.
In this case, the output voltage is given by:
EQUATION 3-1:
VOUT = -|2VIN| + IOROUT
( ) 1
where ROUT = f
1+ 1
C1 C2
+ RSC2
(C2 + COUT)
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:
VRIPPLE = IORRIPPLE
where
RRIPPLE
=
2f(C2
1
+1
+ COUT) 2fCOUT
+ ESR C2
(C2 + COUT)
Here, ESR is the equivalent series resistance of COUT.
| | b.) For regulated operation when VIN >
VR
2
.
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:
VOUTMIN = -|VR| + IOROUT
where ROUT =
1
2fCOUT
+
ESR C2
(C2 + COUT)
The output ripple voltage is given approximately by:
EQUATION 3-4:
( ) (2VIN – |VR| + ESR IOC2
VRIPPLE =
N
1+
C1
1
C2
)
( ) where N =
1 +1
C1 C2
(C2 + COUT)
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 4.7µF for COUT, and values of
0.47µF for C1 and C2, the typical output impedance of
the TC1142 in regulation is 0.5Ω. 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 = 0.1Ω, IOUT = 20mA
C1, C2
(µF)
COUT
(µF)
VIN VOUT
(V) (V)
VRIPPLE
(mV)
0.1
4.7
3.2
-5
0.22
4.7
3.2
-5
0.33
4.7
3.2
-5
0.47
4.7
3.2
-5
0.68
4.7
3.2
-5
1.0
4.7
3.2
-5
0.1
10
3.2
-5
0.22
10
3.2
-5
0.33
10
3.2
-5
0.47
10
3.2
-5
0.68
10
3.2
-5
1.0
10
3.2
-5
14.6
31.4
46.1
63.9
88.7
123.2
7.0
15.1
22.4
31.5
44.7
63.8
DS21360B-page 6
2002 Microchip Technology Inc.
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