Datasheet

FB
Vout
R
FBT
R
FBB
LM22680
SNVS595K –SEPTEMBER 2008–REVISED APRIL 2013 www.ti.com
It is good practice to include a high frequency bypass capacitor as close as possible to the LM22680. 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:
(15)
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.
If the switching frequency is set higher than 500 kHz, the capacitance value may not be reduced proportionally
due to stability requirements. The internal compensation is optimized for circuits with a 500 kHz switching
frequency. See the Internal Loop Compensation section for more details.
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 greater than V
FB
(1.285V typ), a voltage divider is used between the output and the FB pin,
as shown in Figure 22. The following equation can be used to calculate the resistor values of this divider:
(16)
A good value for R
FBB
is 1k Ω. This will help to provide some of the minimum load current requirement and
reduce susceptibility to noise pick-up. The top of R
FBT
should be connected directly to the output capacitor or to
the load for remote sensing. If the divider is connected to the load, a local high-frequency bypass should be
provided at that location.
Figure 22. Resistive Feedback Divider
A maximum value of 10 kΩ is recommended for the sum of R
FBB
and R
FBT
to maintain good output voltage
accuracy.
14 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM22680