UC1846-SP_15 Datasheet, PDF (16/29 Pages) Texas Instruments – Current-Mode PWM Controller

UC1846-SP
SLUS871C – JANUARY 2009 – REVISED OCTOBER 2015
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8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Input Capacitor Selection
Load current, duty cycle, and switching frequency are several factors which determine the magnitude of the input
ripple voltage. Without the input capacitor, the pulsating current of Q1 would need to be completely supplied by
the host source, VIN, which commonly does not have sufficiently low output impedance. Thus there would be
substantial noise on the host dc voltage source and an increase in the conducted EMI on the board. The input
capacitor, CIN, effectively filters the input current so the current from the host dc source is approximately an
average current.
The input ripple voltage amplitude is directly proportional to the output load current. The maximum input ripple
amplitude occurs at maximum output load. Also, the amplitude of the voltage ripple varies with the duty cycle of
the converter.
UC1846-SP requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 47 μF of
effective capacitance on the VIN input voltage pins. In some applications additional bulk capacitance may also be
required for the VIN input. The effective capacitance includes any DC bias effects. The voltage rating of the input
capacitor must be greater than the maximum input voltage. The capacitor must also have a ripple current rating
greater than the maximum input current ripple of the UC1846-SP. The input ripple current can be calculated
using Equation 6.
Icirms = Iout ´
Vout (Vinmin- Vout)
´
Vinmin
Vinmin
(6)
The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the
capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that
is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors
because they have a high capacitance to volume ratio and are fairly stable over temperature. The output
capacitor must also be selected with the DC bias taken into account. The capacitance value of a capacitor
decreases as the DC bias across a capacitor increases.
The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be
calculated using Equation 7.
DVin = ^{Iout max´ 0.25}
Cin ´ fSW
(7)
8.2.1.2.2 Output Capacitor Selection
The output capacitance of a switching regulator is a vital part of the overall feedback system. The energy storage
inductor and the output capacitor form a second-order low-pass filter.
In switching power supply power stages, the function of output capacitance is to store energy. The energy is
stored in the capacitor’s electric field due to the voltage applied. Thus, qualitatively, the function of a capacitor is
to attempt to maintain a constant voltage.
The value of output capacitance of a buck power stage is generally selected to limit output voltage ripple to the
level required by the specification. Since the ripple current in the output inductor is usually already determined,
the series impedance of the capacitor primarily determines the output voltage ripple. The three elements of the
capacitor that contribute to its impedance (and output voltage ripple) are equivalent series resistance (ESR),
equivalent series inductance (ESL), and capacitance (C). The following gives guidelines for output capacitor
selection.
For continuous inductor current mode operation, to determine the amount of capacitance needed as a function of
inductor current ripple, ΔI_L, switching frequency, f_S, and desired output voltage ripple, ΔV_O, Equation 8 is used
assuming all the output voltage ripple is due to the capacitor’s capacitance.
C³
DIL
8 ´ fS ´ DVO
where ΔI_L is the inductor ripple current.
•
(8)
16
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