DCP020503 Datasheet, PDF (9/17 Pages) Texas Instruments – Miniature, 2W, Isolated UNREGULATED DC/DC CONVERTERS

DCP02 Series
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Decoupling Ceramic Capacitors
All capacitors have losses because of internal
equivalent series resistance (ESR), and to a lesser
degree, equivalent series inductance (ESL). Values
for ESL are not always easy to obtain. However,
some manufacturers provide graphs of frequency
versus capacitor impedance. These graphs typically
show the capacitor impedance falling as frequency is
increased (as shown in Figure 10). As the frequency
increases, the impedance stops decreasing and
begins to rise. The point of minimum impedance
indicates the resonant frequency of the capacitor.
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.
Z
X_C
X_L
0
f_O
Frequency
Z = Ö(X_C - X_L)² + (ESR)²
Where:
X_C is the reactance due to the capacitance.
X_L is the reactance due to the ESL.
f_O is the resonant frequency.
Figure 10. Capacitor Impedance vs Frequency
At f_O, X_C = X_L; 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:
V_IN = V_PK – (ESR × I_TR)
(1)
Where:
V_IN is the voltage at the device input.
V_PK is the maximum value of the voltage on the
capacitor during charge.
I_TR 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.
SBVS011K – MARCH 2000 – REVISED FEBRUARY 2008
Input Capacitor and the Effects of ESR
If the input decoupling capacitor is not ceramic with
<20mΩ ESR, then at the instant the power transistors
switch on, the voltage at the input pins falls
momentarily. Should the voltage fall below
approximately 4V, the DCP detects an under-voltage
condition and switches the DCP drive circuits to the
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
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 2.2µF low-ESR
ceramic capacitor ensures a good startup
performance. For the remaining input voltage ranges,
0.47µF 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.
Output Ripple Calculation Example
DCP020505: Output voltage 5V, Output current 0.4A.
At full output power, the load resistor is 12.5Ω. Output
capacitor of 1µF, ESR of 0.1Ω. Capacitor discharge
time 1% of 800kHz (ripple frequency):
t_DIS = 0.0125µs
τ = C × R_LOAD
τ = 1 × 10^-6 × 12.5 = 12.5µs
V_DIS = V_O(1 – EXP(–t_DIS/τ))
V_DIS = 5mV
By contrast, the voltage dropped because of ESR:
V_ESR = I_LOAD × ESR
V_ESR = 40mV
Ripple voltage = 45mV
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