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Discussion and Component Selection

Figure 1. Schematics

2 Discussion and Component Selection

On this evaluation board, the LM2661 is used in the doubling configuration. This configuration uses only

two capacitors and one diode. There is a manual shutdown jumper designated as J1 included. The

internal oscillator is used with a frequency of 80 kHz. In doubling mode, the oscillator frequency can only

be modified using an external capacitor and cannot be driven by an external clock. The Schottky diode D1

is needed only for start-up but should be able to handle the current required to charge the output capacitor

(I=C*dV/dt). An MBR0520LT1 20 V, 0.5A diode is used on this board. Capacitor selection is very

important. The capacitors chosen determine the output voltage ripple as well as the output resistance

(Equation 1 and Equation 2). From these equations it is easy to see how capacitor value and ESR help

determine the output resistance and output voltage ripple. For this circuit Taiyo-Yuden type 22µF ceramic

capacitors are used (Model EMK432BJ226MM). These capacitors are used because of their low ESR as

well as stable temperature and frequency characteristics. Therefore, they enhance the parts performance.

Tantalum and ceramic capacitors and other values may be used as well to fit different performance, size,

or cost requirements. Universal pads have been put on the evaluation board so that the capacitors can be

replaced with those of a different size.

The LM2663 is configured as an inverter on this board. A manual shutdown is included and designated J2.

The internal oscillator frequency of 150 kHz is used. The capacitor selection here is important as well

since the output resistance and voltage ripple equations are the same as they are for the LM2661

(Equation 1 and Equation 2). This circuit runs at a higher frequency than the LM2661 so smaller capacitor

values can be used. For this circuit Taiyo-Yuden type 10µF ceramic capacitors are used (Model

JMK316BJ106ML-T). Once again the low ESR and stable characteristics of these capacitors are the

reasons they were chosen. Other types and sizes of capacitors may be used here as well for different

performance, size, or cost requirements.

The LM2664 is also used as an inverter on this board. A manual shutdown designated as J3 is included.

The LM2664 does not have an adjustable frequency; it is fixed at 160 kHz. This circuit has the same

equations for output resistance and output voltage ripple as the previous two circuits and the capacitor

selection is once again important (Equation 1 and Equation 2). Taiyo Yuden multi-layer ceramic chip 3.3

µF capacitors are used for this circuit (Model LMK316BJ335ML-T). These capacitors are chosen for their

low ESR (measured ≊ 25 mΩ) and small (1206) case size. They show the high performance of the

LM2664 as well as the small size for the complete circuit. The output voltage ripple was measured to be

less than 75mV peak to peak with a 40 mA load. Again other types and sizes of capacitors may be used

for different performance and/or size requirements.

SNVA014C–November 1999–Revised April 2013 AN-1142 LM2661/3/4 Evaluation Board

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