Datasheet

I
OUT
x D
C
IN
x F
S
V
PPIN
=
+ I
OUT
x ESR
CIN
P
D_CIN
= I
2
RMS_CIN
x R
ESR_CIN
V
OUT
(V
IN
- V
OUT
)
I
RMS_CIN
= I
OUT
V
IN
LM10506
www.ti.com SNVS729E –SEPTEMBER 2011–REVISED MARCH 2013
The device is designed to be used with ceramic capacitors on the outputs of the buck regulators. The
recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper
tolerances over voltage and temperature. The recommended value for the output capacitors is 22 μF, 6.3V with
an ESR of 2mΩ or less. The output capacitors need to be mounted as close as possible to the output/ground
pins of the device.
Table 4. Recommended Output Capacitors
Type
Model Vendor Voltage Rating Case Size
Vendor
08056D226MAT2A Ceramic, X5R AVX Corporation 6.3V 0805, (2012)
C0805L226M9PACTU Ceramic, X5R Kemet 6.3V 0805, (2012)
ECJ-2FB0J226M Ceramic, X5R Panasonic - ECG 6.3V 0805, (2012)
JMK212BJ226MG-T Ceramic, X5R Taiyo Yuden 6.3V 0603, (1608)
C2012X5R0J226M Ceramic, X5R TDK Corporation 6.3V 0603, (1608)
Input Capacitor Selection
There are 3 buck regulators in the LM10506 device. Each of these buck regulators has its own input capacitor
which should be located as close as possible to their corresponding SWx_VIN and SWx_GND pins, where x
designates Buck 1, 2 or 3. The 3 buck regulators operate at 120° out of phase, which means that they switch on
at equally spaced intervals, in order to reduce the input power rail ripple. It is recommended to connect all the
supply/ground pins of the buck regulators, SWx_VIN to two solid internal planes located under the device. In this
way, the 3 input capacitors work together and further reduce the input current ripple. A larger tantalum capacitor
can also be located in the proximity of the device.
The input capacitor supplies the AC switching current drawn from the switching action of the internal power
FETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the input capacitor
is large. The input capacitor must be rated to handle both the RMS current and the dissipated power.
The input capacitor must be rated to handle this current:
(6)
The power dissipated in the input capacitor is given by:
(7)
The device is designed to be used with ceramic capacitors on the inputs of the buck regulators. The
recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper
tolerances over voltage and temperature. The minimum recommended value for the input capacitor is 10 µF with
an ESR of 10 mΩ or less. The input capacitors need to be mounted as close as possible to the power/ground
input pins of the device.
The input power source supplies the average current continuously. During the PFET switch on-time, however,
the demanded di/dt is higher than can be typically supplied by the input power source. This delta is supplied by
the input capacitor.
A simplified “worst case” assumption is that all of the PFET current is supplied by the input capacitor. This will
result in conservative estimates of input ripple voltage and capacitor RMS current.
Input ripple voltage is estimated as follows:
where
• V
PPIN
: estimated peak-to-peak input ripple voltage,
• I
OUT
: Output Current
• C
IN
: Input capacitor value
• ESR
CIN
: input capacitor ESR. (8)
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