Decoupling Ceramic Capacitors Input Capacitor and the Effects of ESR
0
f
O
Frequency
X
L
Z
Where:
X isthereactanceduetothecapacitance.
C
X isthereactanceduetotheESL.
f istheresonantfrequency.
L
O
Z= (X X )Ö -
C L
2 2
+(ESR)
X
C
DCP02 Series
SBVS011K – MARCH 2000 – REVISED FEBRUARY 2008
All capacitors have losses because of internal If the input decoupling capacitor is not ceramic with
equivalent series resistance (ESR), and to a lesser <20m Ω ESR, then at the instant the power transistors
degree, equivalent series inductance (ESL). Values switch on, the voltage at the input pins falls
for ESL are not always easy to obtain. However, momentarily. Should the voltage fall below
some manufacturers provide graphs of frequency approximately 4V, the DCP detects an under-voltage
versus capacitor impedance. These graphs typically condition and switches the DCP drive circuits to the
show the capacitor impedance falling as frequency is off state. This detection is carried out as a precaution
increased (as shown in Figure 10 ). As the frequency against a genuine low input voltage condition that
increases, the impedance stops decreasing and could slow down or even stop the internal circuits
begins to rise. The point of minimum impedance from operating correctly. A slow-down or stoppage
indicates the resonant frequency of the capacitor. would result in the drive transistors being turned on
This frequency is where the components of too long, causing saturation of the transformer and
capacitance and inductance reactance are of equal destruction of the device.
magnitude. Beyond this point, the capacitor is not
Following detection of a low input voltage condition,
effective as a capacitor.
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 µ F low-ESR
ceramic capacitor ensures a good startup
Figure 10. Capacitor Impedance vs Frequency
performance. For the remaining input voltage ranges,
0.47 µ F ceramic capacitors are recommended.
Tantalum capacitors are not recommended, since
At f
O
, X
C
= X
L
; however, there is a 180 ° phase
most do not have low-ESR values and will degrade
difference resulting in cancellation of the imaginary
performance. If tantalum capacitors must be used,
component. The resulting effect is that the impedance
close attention must be paid to both the ESR and
at the resonant point is the real part of the complex
voltage as derated by the vendor.
impedance; namely, the value of the ESR. The
resonant frequency must be well above the 800kHz
Output Ripple Calculation Example
switching frequency of the DCP and DCVs.
DCP020505: Output voltage 5V, Output current 0.4A.
The effect of the ESR is to cause a voltage drop
At full output power, the load resistor is 12.5 Ω . Output
within the capacitor. The value of this voltage drop is
capacitor of 1 µ F, ESR of 0.1 Ω . Capacitor discharge
simply the product of the ESR and the transient load
time 1% of 800kHz (ripple frequency):
current, as shown:
t
DIS
= 0.0125 µ s
V
IN
= V
PK
– (ESR × I
TR
) (1)
τ = C × R
LOAD
Where:
τ = 1 × 10
-6
× 12.5 = 12.5 µ s
V
IN
is the voltage at the device input.
V
DIS
= V
O
(1 – EXP( – t
DIS
/ τ ))
V
PK
is the maximum value of the voltage on the
V
DIS
= 5mV
capacitor during charge.
By contrast, the voltage dropped because of ESR:
I
TR
is the transient load current.
V
ESR
= I
LOAD
× ESR
The other factor that affects the performance is the
V
ESR
= 40mV
value of the capacitance. However, for the input and
Ripple voltage = 45mV
the full wave outputs (single-output voltage devices),
ESR is the dominant factor.
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