ADVANCED INFORMATION
NOISE CONSIDERATIONS
TK654xx
GENERAL
Because of the switching waveforms inherent to the operation of DC-DC converters, they tend to be noisier than their
linear counterparts. A DC-DC converter uses switching techniques to do the power conversion at higher efficiencies than
a linear regulator at the price of noisier output ripple. The TK654xx is no exception to this, even though the proprietary
architecture chosen was centered in providing the user fast transient response, low quiescent current and minimum
ripple.
INPUT/OUTPUT CAPACITORS (GENERAL)
By the nature of their operation, DC-DC converters generate large switching currents on both the input and output of the
circuit. The input and output capacitors should be as large as practical with a low Equivalent Series Resistance (ESR).
Since low temperatures cause the value of capacitance to decrease and the ESR to increase, care should be taken to
select capacitors that have acceptable characteristics over the desired operating temperature range.
INPUT CAPACITOR
The function of the input capacitor is to reduce the supply impedance and to provide sufficient input current during
switching for stable circuit operation. The input capacitor should be physically located as close as posssible to the
converter to minimize the lead inductance and to reduce the impedance of the source at high frequencies. By having
the capacitor close to the converter, the switching current pulses are supplied locally by the capacitor instead of running
across the printed circuit board by long etch runs. This greatly reduces the noise on the board.
OUTPUT CAPACITOR
The function of the output capacitor is to reduce the ripple voltage appearing on the converter output. The output ripple
voltage is the AC voltage which appears on the regulated DC output. It is inherent in all DC-DC converters and is the
result of the conversion of the input DC to AC and then back to output DC. The output ripple is the result of two factors,
which are 90° out of phase. The first factor is the result of the change in the stored charge of the output capacitor as it
is charged by the switching current from the converter and then discharged by the load current. This factor determines
the value (µF) of the output capacitor. The second factor is the product of the capacitor’s charge/discharge current times
its ESR. This factor determines the requirement for low ESR capacitors. In DC-DC converters, the ripple produced by
the capacitor ESR is often larger than the ripple produced by the change in charge. For this reason, high quality ceramic,
tantalum, or aluminum filter capacitors are required to minimize the output ripple. This contribution to output ripple and
noise can also be minimized by the addition of a small (0.01 µF to 0.1 µF) ceramic capacitor in parallel with the bulk output
capacitor. Adding a Pi filter as shown below (Figure 4) can further reduce output noise. The Pi filter has a tendency to
reduce the efficiency by a couple of percentage points and yet the noise improves by 20 dB/decade.
LBO
OUT
SW
LBI
V
GND
V
IN
V
= 5 V
IN
V
OUT
L
1
1Ω
100 µH
D
22 µF
C
1
1
LL 103
22 µF
OPTIONAL
FILTER
FIGURE 4: OPTIONAL PI FILTER
September 1999 TOKO, Inc.
Page 15
TOKO [ TOKO, INC ]
