DCP01B datasheet(13/21 Pages) TI

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DCP01B SERIES

SBVS012C − DECEMBER 2000 − REVISED AUGUST 2005

www.ti.com

Decoupling Ceramic Capacitors

All capacitors have losses due to their internal equivalent

series resistance (ESR), and to a lesser degree their

equivalent series inductance (ESL). Values for ESL are

not always easy to obtain. However, some manufacturers

provide

graphs

Frequency

versus

Capacitor

Impedance. These will show the capacitors’ impedance

falling as frequency is increased (see Figure 4). As the

frequency

increased,

the

impedance

will

stop

decreasing and begin to rise. The point of minimum

impedance indicates the capacitors’ resonant frequency.

This frequency is where the components of capacitance

and inductance reactance are of equal magnitude. Beyond

this point, the capacitor is not effective as a capacitor.

Frequency

Where:

XC is the reactance due to the capacitance,

XL is the reactance due to the ESL

O the resonant frequency

√(XC − XL)2+(ESR)2

Figure 4. Capacitor Impedance vs Frequency

At fO, XC = XL; however, there is a 180° phase difference

resulting in cancellation of the imaginary component. The

resulting effect is that the impedance at the resonant point

is the real part of the complex impedance; namely, the

value of the ESR. The resonant frequency must be well

above the 800kHz switching frequency of the DCP and

DCVs.

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):

+ V

* (ESR

TR)

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), the ESR is the

dominant factor.

Input Capacitor and the effects of ESR

If the input decoupling capacitor is not ceramic with

< 20m

Ω ESR, then at the instant the power transistors

switch on, the voltage at the input pins will fall momentarily.

Should the voltage fall below approximately 4V, the DCP

will detect an under-voltage condition and switch the DCP

drive circuits to the off state. This 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. This would result 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. Then the device tries to

restart. If the input capacitor is still unable to maintain the

input voltage, shutdown recurs. This process is repeated

until the capacitor is charged sufficiently to start the device

correctly. Otherwise, the device will be caught up in a loop.

Normal startup should occur in approximately 1ms from

power being applied to the device. If a considerably longer

startup duration time is encountered, it is likely that either

(or both) the input supply or the capacitors are not

performing adequately.

For 5V to 15V input devices, a 2.2

µF low-ESR ceramic

capacitor will ensure a good startup performance, and for

the remaining input voltage ranges, 0.47

µF ceramic

capacitors are good. 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.

Output Ripple Calculation Example

DCP020505: Output voltage 5V, Output current 0.4A. At

full output power, the load resistor is 12.5

Ω. Output

capacitor of 1

µF, ESR of 0.1Ω. Capacitor discharge time

1% of 800kHz (ripple frequency):

tDIS = 0.0125µs

t = C

× RLOAD

t = 1

× 10−6 × 12.5 = 12.5µs

VDIS = VO(1 − EXP(−tDIS/τ))

VDIS = 5mV

By contrast the voltage dropped due to the ESR:

VESR = ILOAD × ESR

VESR = 40mV

Ripple voltage = 45mV

Clearly, increasing the capacitance will have a much

smaller effect on the output ripple voltage than reducing

the value of the ESR for the filter capacitor.

(1)

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