Datasheet NCP1729 (ON Semiconductor) - 6

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NCP1729 Capacitor Selection
Vout to GND, it is recommended that a large value capacitor In order to maintain the lowest output resistance and (at least equal to C1) be connected from Vin to GND. If the output ripple voltage, it is recommended that low ESR device is loaded from Vin to Vout, a small (0.7 F) capacitor capacitors be used. Additionally, larger values of C1 will between the pins is sufficient. lower the output resistance and larger values of C2 will reduce output voltage ripple. (See Equation 3).
Voltage Inverter
Table 1 shows various values of C The most common application for a charge pump is the 1, C2 and C3 with the corresponding output resistance values at 25°C. Table 2 voltage inverter (Figure 14). This application uses two or shows the output voltage ripple for various values of C three external capacitors. The C 1, C2 1 (pump capacitor) and C2 and C (output capacitor) are required. The input bypass capacitor 3. The data in Tables 1 and 2 was measured not calculated. C3, may be necessary depending on the application. The output is equal to −Vin plus any voltage drops due to loading. Refer to Tables 1 and 2 for capacitor selection. The test setup
Table 1. Output Resistance vs. Capacitance
used for the majority of the characterization is shown in
(C1 = C2 = C3), Vin = 4.75 V and Vout = −4.0 V
Figure 14.
C1 = C2 = C3 ( F) Rout ( ) Layout Considerations
0.68 55.4 As with any switching power supply circuit, good layout 1.3 36.9 practice is recommended. Mount components as close 3.3 26.0 together as possible to minimize stray inductance and 7.3 25.8 capacitance. Also use a large ground plane to minimize noise leakage into other circuitry. 10 25.5 24 25.0
Capacitor Resources
Selecting the proper type of capacitor can reduce 50 24.0 switching loss. Low ESR capacitors are recommended. The NCP1729 was characterized using the capacitors listed in
Table 2. Output Voltage Ripple vs. Capacitance
Table 3. This list identifies low ESR capacitors for the
(C1 = C2 = C3), Vin = 4.75 V and Vout = −4.0 V
voltage inverter application.
C1 = C2 = C3 Output Voltage Ripple ( F) (mV) Table 3. Capacitor Types
0.68 322
Manufacturer/Contact Part Types/Series
1.3 205
AVX AV
TPS 843−448−9411 3.3 120 www.avxcorp.com 7.3 69
Cornell Dubilier Cornell Dubilier
ESRD 10 56 508−996−8561 www ll d bili .cornell−dubilier.com 24 32
Sanyo/Os−con Sanyo/Os−con
SN 50 20 619−661−6835 SVP www id / ht .sanyovideo.com/oscon.htm
Input Supply Bypassing Visha V y ishay
593D The input voltage, Vin should be capacitively bypassed to 603−224−1961 594 reduce AC impedance and minimize noise effects due to the www i h .vishay.com switching internals in the device. If the device is loaded from
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