TC1121 Datasheet, PDF (5/14 Pages) Microchip Technology – 100mA Charge Pump Voltage Converter with Shutdown

3.0 APPLICATIONS
3.1 Negative Voltage Converter
The TC1121 is typically used as a charge-pump voltage
inverter. C1 and C2 are the only two external capacitors
used in the operating circuit (Figure 3-1).
FIGURE 3-1:
CHARGE PUMP
INVERTER
.4V to 5.5V
C
N
AP
SC
C1121
1
ND
HDN
SHDN*
AP
UT
UT
2
SHDN should be tied to V _N if not used.
The TC1121 is not sensitive to load current changes,
although its output is not actively regulated. A typical
output source resistance of 11.8Ω means that an input
of +5V results in -5V output voltage under light load,
and only decreases to -3.8V typ with a 100mA load.
The supplied output current is from capacitor C2 during
one-half the charge-pump cycle. This results in a
peak-to-peak ripple of:
V_RIPPLE = I_OUT/2(f_PUMP) (C2) + I_OUT (ESR_C2)
Where f_PUMP is 5kHz (one half the nominal 10kHz
oscillator frequency), and C2 = 150µF with an ESR of
0.2Ω, ripple is about 90mV with a 100mA load current.
If C2 is raised to 390µF, the ripple drops to 45mV.
TC1121
3.2 Changing Oscillator Frequency
The TC1121’s clock frequency is controlled by four
modes:
TABLE 3-1: OSCILLATOR FREQUENCY
MODES
FC
Open
FC = V⁺
Open or
FC = V⁺
Open
OSC
Open
Open
External Capacitor
External Clock
Oscillator Frequency
10kHz
200kHz
See Typical Operating
Characteristics
External Clock Frequency
The oscillator runs at 10kHz (typical) when FC and
OSC are not connected. The oscillator frequency is
lowered by connecting a capacitor between OSC and
GND, but FC can still multiply the frequency by 20
times in this mode.
An external clock source that swings within 100mV of
V⁺ and GND may overdrive OSC in the inverter mode.
OSC can be driven by any CMOS logic output. When
OSC is overdriven, FC has no effect.
Note that the frequency of the signal appearing at
CAP⁺ and CAP^– is half that of the oscillator. In addition,
by lowering the oscillator frequency, the effective
output resistance of the charge-pump increases. To
compensate for this, the value of the charge-pump
capacitors may be increased.
Because the 5kHz output ripple frequency may be low
enough to interfere with other circuitry, the oscillator
frequency can be increased with the use of the FC pin
or an external oscillator. The output ripple frequency is
half the selected oscillator frequency. Although the
TC1121’s quiescent current will increase if the clock
frequency is increased, it allows smaller capacitance
values to be used for C1 and C2.
3.3 Capacitor Selection
In addition to load current, the following factors affect
the TC1121 output voltage drop from its ideal value 1)
output resistance, 2) pump (C1) and reservoir (C2)
capacitor ESRs and 3) C1 and C2 capacitance.
The voltage drop is the load current times the output
resistance. The loss in C2 is the load current times C2’s
ESR; C1’s loss is larger because it handles currents
greater than the load current during charge-pump
operation. Therefore, the voltage drop due to C1 is
about four times C1’s ESR multiplied by the load
current, and a low (or high) ESR capacitor has a
greater impact on performance for C1 than for C2.
In general, as the TC1121’s pump frequency increases,
capacitance values needed to maintain comparable
ripple and output resistance diminish proportionately.
 2002 Microchip Technology Inc.
DS21358B-page 5