The effect of the ESR is to cause a voltage drop within the capacitor. The value of this voltage drop is simply the

product of the ESR and the transient load current, as shown in Equation 1.

VIN = VPK – (ESR × ITR)

(1)

where

• VIN is the voltage at the device input

• VPK is the maximum value of the voltage on the capacitor during charge

• ITR is the transient load current

The other factor that affects the performance is the value of the capacitance. However, for the input and the full

wave outputs (single-output voltage devices), ESR is the dominant factor.

8.2.6 Input Capacitor and the Effects of ESR

If the input decoupling capacitor is not ceramic (and has an ESR greater than 20 mΩ), then at the instant the

power transistors switch on, the voltage at the input pins falls momentarily. If the voltage falls below

approximately 4 V, the device detects an undervoltage condition and switches the internal drive circuits to a

momentary off state. This detection is carried out as a precaution against a genuine low input voltage condition

that could slow down or even stop the internal circuits from operating correctly. A slow-down or stoppage results

in the drive transistors being turned on too long, causing saturation of the transformer and destruction of the

device.

Following detection of a low input voltage condition, the device switches off the internal drive circuits until the

input voltage returns to a safe value, at which time the device tries to restart. If the input capacitor is still unable

to maintain the input voltage, shutdown recurs. This process repeats until the input capacitor charges sufficiently

to start the device correctly.

Normal start-up should occur in approximately 1 ms after power is applied to the device. If a considerably longer

start-up duration time is encountered, it is likely that either (or both) the input supply or the capacitors are not

performing adequately.

For 5-V to 15-V input devices, a 2.2-μF, low-ESR ceramic capacitor ensures good startup performance. For 24-V

input voltage devices, 0.47-μF ceramic capacitors are recommended. Tantalum capacitors are not

recommended, since most do not have low-ESR values and will degrade performance. If tantalum capacitors

must be used, close attention must be paid to both the ESR and voltage as derated by the vendor.

Note

During the start-up period, these devices may draw maximum current from the input supply. If

the input voltage falls below approximately 4 V, the devices may not start up. Connect a 2.2-μF

ceramic capacitor close to the input pins.

8.2.7 Ripple and Noise

A good quality, low-ESR ceramic capacitor placed as close as practical across the input reduces reflected ripple

and ensures a smooth start-up.

A good quality, low-ESR ceramic capacitor placed as close as practical across the rectifier output terminal and

output ground gives the best ripple and noise performance. See application report DC-to-DC Converter Noise

Reduction for more information on noise rejection.

8.2.7.1 Output Ripple Calculation Example

The following example shows that increasing the capacitance has a much smaller effect on the output ripple

voltage than does reducing the value of the ESR for the filter capacitor.

To calculate the output ripple for a DCP010505 device:

• VOUT = 5 V

• IOUT = 0.2 A

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SBVS012I – DECEMBER 2000 – REVISED SEPTEMBER 2020

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