TL7660_14 Datasheet, PDF (9/23 Pages) Texas Instruments – CMOS VOLTAGE CONVERTER

www.ti.com
Typical Applications
TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
Simple Negative Voltage Converter
The majority of applications will undoubtedly utilize the TL7660 for generation of negative supply voltages.
Figure 3 shows typical connections to provide a negative supply negative (GND) for supply voltages below
3.5 V.
V+
10 µF ⁺
-
1
8
2
7
TL7660
3
6
4
5
10 µF ^-
+
V_OUT = –V+
Figure 3. Simple Negative-Voltage Converter
The output characteristics of the circuit in Figure 3 can be approximated by an ideal voltage source in series with
a resistance. The voltage source has a value of –V_CC. The output impedance (R_O) is a function of the ON
resistance of the internal MOS switches (shown in Figure 2), the switching frequency, the value of C₁ and C₂,
and the ESR (equivalent series resistance) of C₁ and C₂. A good first order approximation for R_O is:
R_O ≈ 2(R_SW1 + R_SW3 + ESR_C1) + 2(R_SW2 + R_SW4 + ESR_C1)
R_O ≈ 2(R_SW1 + R_SW3 + ESR_C1) + 1/f_PUMPC₁ + ESR_C2
Where f_PUMP = f_OSC/2 , R_SWX = MOSFET switch resistance.
Combining the four RSWX terms as RSW, we see that:
R_O ≈ 2 (R_SW) + 1/f_PUMPC₁ + 4 (ESR_C1) + ESR_C2
R_SW, the total switch resistance, is a function of supply voltage and temperature (See the Output Source
Resistance graphs). Careful selection of C₁ and C₂ reduces the remaining terms, minimizing the output
impedance. High value capacitors reduce the 1/f_PUMPC₁ component, and low ESR capacitors lower the ESR
term. Increasing the oscillator frequency reduces the 1/f_PUMPC₁ term but may have the side effect of a net
increase in output impedance when C₁ > 10 µF and there is no longer enough time to fully charge the capacitors
every cycle. In a typical application where f_OSC = 10 kHz and C = C1 = C2 = 10 µF:
R_O ≈ 2(23) + 1/(5 × 10³)(10^–5) + 4(ESR_C1) + ESR_C2
R_O ≈ 46 + 20 + 5 (ESR_C)
Because the ESRs of the capacitors are reflected in the output impedance multiplied by a factor of 5, a high
value could potentially swamp out a low 1/f_PUMPC₁ term, rendering an increase in switching frequency or filter
capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10 Ω.
Submit Documentation Feedback
9