Datasheet

LM22675
www.ti.com SNVS591K –SEPTEMBER 2008–REVISED MARCH 2013
INPUT CAPACITOR
The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current
during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are
desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry,
sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for
determining the input voltage ripple is shown below:
(12)
Where V
ri
is the peak-to-peak ripple voltage at the switching frequency. Another concern is the RMS current
passing through this capacitor. The following equation gives an approximation to this current:
(13)
The capacitor must be rated for at least this level of RMS current at the switching frequency.
All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature
coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum
capacitance up to the desired value. This may also help with RMS current constraints by sharing the current
among several capacitors. Many times it is desirable to use an electrolytic capacitor on the input, in parallel with
the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply caused by
long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input
voltage rating of the LM22675.
It is good practice to include a high frequency bypass capacitor as close as possible to the LM22675. This small
case size, low ESR, ceramic capacitor should be connected directly to the VIN and GND pins with the shortest
possible PCB traces. Values in the range of 0.47 µF to 1 µF are appropriate. This capacitor helps to provide a
low impedance supply to sensitive internal circuitry. It also helps to suppress any fast noise spikes on the input
supply that may lead to increased EMI.
OUTPUT CAPACITOR
The output capacitor is responsible for filtering the output voltage and supplying load current during transients.
Capacitor selection depends on application conditions as well as ripple and transient requirements. Best
performance is achieved with a parallel combination of ceramic capacitors and a low ESR SP™ or POSCAP™
type. Very low ESR capacitors such as ceramics reduce the output ripple and noise spikes, while higher value
electrolytics or polymer provide large bulk capacitance to supply transients. Assuming very low ESR, the
following equation gives an approximation to the output voltage ripple:
(14)
Typically, a total value of 100 µF, or greater, is recommended for output capacitance.
In applications with V
out
less than 3.3V, it is critical that low ESR output capacitors are selected. This will limit
potential output voltage overshoots as the input voltage falls below the device normal operating range.
BOOT-STRAP CAPACITOR
The bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the N-
channel MOSFET. The recommended value of this capacitor is 10 nF and should be a good quality, low ESR
ceramic capacitor. In some cases it may be desirable to slow down the turn-on of the internal power MOSFET, in
order to reduce EMI. This can be done by placing a small resistor in series with the C
boot
capacitor. Resistors in
the range of 10Ω to 50Ω can be used. This technique should only be used when absolutely necessary, since it
will increase switching losses and thereby reduce efficiency.
OUTPUT VOLTAGE DIVIDER SELECTION
For output voltages between about 1.285V and 5V, the -ADJ option should be used, with an appropriate voltage
divider as shown in Figure 16. The following equation can be used to calculate the resistor values of this divider:
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