TC3682 Datasheet, PDF (5/10 Pages) Microchip Technology – Inverting Charge Pump Voltage Doublers with Active Low Shutdown

TC3682/TC3683/TC3684
4.0 APPLICATIONS INFORMATION
4.1 Output Voltage Considerations
The TC3682/TC3683/TC3684 perform inverting
voltage conversions but do not provide any type of
regulation. The output voltage will droop in a linear
manner with respect to the output load current. The
value of the equivalent output resistance of the ‘-V_IN’
output is approximately 50Ω nominal at +25°C and
V_IN = +5V. The value of the ‘-2V_IN’ output is approxi-
mately 140Ω nominal at +25°C and V_IN = +5V. In this
particular case, ‘-V_IN’ is approximately -5V and ‘-2V_IN’
is approximately -10V at very light loads and each
stage will droop according to the equation below:
V_DROOP = I_OUT x R_OUT
4.2 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 C_OUT will reduce output ripple.
Note: For proper charge pump operation, C1
must have a voltage rating greater than or
equal to V_IN, while C2 and C_OUT must have
a voltage rating greater than or equal to
2V_IN.
Table 4-1 shows various values of C1/C2 and the
corresponding output resistance values for V_IN = 5V
@ +25°C.
Table 4-2 shows the output voltage ripple for various
values of C_OUT (again assuming V_IN = 5V @ +25°C).
The V_RIPPLE values assume a 1mA output load current
and a 0.1Ω ESR_COUT.
TABLE 4-1: OUTPUT RESISTANCE
VS. C1/C2 (ESR = 0.1Ω)
C1, C2
(µF)
0.33
1
3.3
TC3682
R_OUT(Ω)
633
262
120
TC3683
R_OUT(Ω)
184
120
95
TC3684
R_OUT(Ω)
120
102
84
TABLE 4-2:
OUTPUT VOLTAGE RIPPLE
VS. C_OUT (ESR = 0.1Ω)
I_OUT = 1mA
C_OUT
(µF)
TC3682
V_RIPPLE
(mV)
TC3683
V_RIPPLE
(mV)
TC3684
V_RIPPLE
(mV)
0.33
192
60
27
1
63
21
16
3.3
17
8
7
4.3 Input Supply Bypassing
The V_IN input should be capacitively bypassed to
reduce AC impedance and minimize noise effects due
to the switching internal to the device. It is
recommended that a large value capacitor (at least
equal to C1) be connected from V_IN to GND for optimal
circuit performance.
4.4 Shutdown Input
The TC3682/TC3683/TC3684 is enabled when SHDN
is high, and disabled when SHDN is low. This input
cannot be allowed to float. (If SHDN is not required, see
the TC2682/TC2683/TC2684 data sheet.) The SHDN
input should be limited to 0.3V above V_IN.
4.5 Inverting Voltage Doubler
The most common application for the TC3682/TC3683/
TC3684 devices is the inverting voltage doubler
(Figure 4-1). This application uses three external
capacitors: C1, C2 and C_OUT.
Note: A power supply bypass capacitor is
recommended.
The output is equal to -2V_IN plus any voltage drops due
to loading. Refer to Table 4-1 and Table 4-2 for
capacitor selection guidelines.
FIGURE 4-1:
INVERTING VOLTAGE
DOUBLER TEST CIRCUIT
V_IN
+
C_IN
+
C1
7 _C1+
68
V_{IN SHDN}
1 _{C1– TC3682}
+
2 _C2+
TC3683
TC3684
C2
-2V_IN ⁴
3 _C2–
GND
C_OUT
+ ^RL
5
V_OUT
Device
TC3682
TC3683
TC3684
C_IN
3.3µF
1µF
0.33µF
C1
3.3µF
1µF
0.33µF
C2
3.3µF
1µF
0.33µF
C_OUT
3.3µF
1µF
0.33µF
4.6 Layout Considerations
As with any switching power supply circuit, good layout
practice is recommended. Mount components as close
together as possible to minimize stray inductance and
capacitance. Also use a large ground plane to minimize
noise leakage into other circuitry.
 2002 Microchip Technology Inc.
DS21556B-page 5