Datasheet
SLVS289A − MARCH 2000 − REVISED OCTOBER 2000
26
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
output capacitor RMS current
Assuming the inductor ripple current totally goes through the output capacitor to ground, the RMS current in the
output capacitor can be calculated as:
12
I
I
∆
=
O(rms)
Where I
O(rms)
is the maximum RMS current in the output capacitor (A); ∆I is the peak-to-peak inductor ripple
current (A).
Example:
∆I = 1 A, so I
O(rms)
= 0.29 A
input capacitor RMS current
The input capacitor RMS current is important for input capacitor design. Assuming the input ripple current totally
goes into the input capacitor to the power ground, the RMS current in the input capacitor can be calculated as:
I
I(rms)
+ I
O
2
D
(
1 * D
)
)
1
12
D I
ripple
2
Ǹ
Where I
I(rms)
is the input RMS current in the input capacitor (A); I
O
is the output current (A); I
ripple
is the
peak-to-peak output inductor ripple current; D is the duty cycle. From the equation, it can be seen that the
highest input RMS current usually occurs at the lowest input voltage, so it is the worst case design for input
capacitor ripple current.
Example:
I
O
= 5 A; D = 0.36; I
ripple
= 1 A,
Then, I
I(rms)
= 2.46 A
layout and component value consideration
Good power supply results will only occur when care is given to proper design and layout. Layout and
component value will affect noise pickup and generation and can cause a good design to perform with less than
expected results. With a range of current from milliamps to tens or even hundreds of amps, good power supply
layout and component selection, especially for a fast ripple controller, is much more difficult than most general
PCB design. The general design should proceed from the switching node to the output, then back to the driver
section, and, finally, to placing the low-level components. In the following list are several specific points to
consider before layout and component selection for TPS56302:
1. All sensitive analog components should be referenced to ANAGND. These include components connected
to SLOWST, DROOP, IOUT, OCP, VSENSE, VREFB, VHYST, BIAS, and LOSENSE/LOHIB.
2. The input voltage range for TPS56302 is low from 2.8-V to 5.5-V, so it has a voltage tripler (charge pump)
inside to deliver proper voltage for internal circuitry. To avoid any possible noise coupling, a low ESR
capacitor on V
CC
is recommended.
3. For the same reason in Item 2, the ANAGND and DRVGND should be connected as close as possible to
the IC.
4. The bypass capacitor should be placed close to the TPS56302.