l6996dtr STMicroelectronics, l6996dtr Datasheet - Page 18

• Manufacturer Part Number: l6996dtr
• Description: Dinamically Programmable Synchronous Step Down Controller For Mobile Cpus
• Manufacturer: STMicroelectronics

L6996
Table 2. PCB Layout guidelines
3
3.1 V
In this design it is considered a low profile demoboard, so a great attention is given to the components height.
3.2 Input capacitor
A pulsed current (with zero average value) flows through the input capacitor of a buck converter. The AC com-
ponent of this current is quite high and dissipates a considerable amount of power on the ESR of the capacitor:
Eq 15
The I
Eq 16
Neglecting the last term, the equation reduces to:
Eq 17
P
therefore, should be selected for a RMS ripple current rating as high as half the respective maximum output
current. Electrolytic capacitors are the most used because are the cheapest ones and are available with a wide
range of RMS current ratings. The only drawback is that, considering a requested ripple current rating, they are
physically larger than other capacitors. Very good tantalum capacitors are coming available, with very low ESR
and small size. The only problem is that they occasionally can burn out if subjected to very high current during
the charge. So, it is better avoid this type of capacitors for the input filter of the device. In fact, they can be sub-
jected to high surge current when connected to the power supply. If available for the requested value and volt-
age rating, the ceramic capacitors have usually a higher RMS current rating for a given physical size (due to the
very low ESR). From the equation 17 it is found:
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Low radiation and low magnetic coupling with the
adjacent circuitry
Don’t penalty the efficiency
Ensure high accuracy in the current sense system
Reduce the noise effects on IC
CIN
, and also I
DESIGN EXAMPLES
RMS
IN
= 20V I
current is given by:
CINRMS
OUT
= 23A
, has a maximum equal to I
Goal
Icin
Icin
P
CIN
rm s
rm s
=
=
=
ESR
Io ut
Iout
CIN
2
Iou t
1 –
1 –
OUT
2
Keep the power traces and load connections short and wide.
1) Small switching current loop areas. (For example Placing
2) Controller placed as close as possible to the Power
3) Group the gate drive component (Boot cap and diode
Cs+, CS- traces must be made by Kelvin connection. Also the
traces should be separated from the power plane by a ground
plane, run parallel.
1) Put the feedback component (like the VP network as close
2) The feedback connection (like the FB trace, or CS+/CS-
3) Make the controller ground connection like in the figure 16.
Vin
----------------------------------------------- -
/2 (@ V
C
close as possible each to others).
MOSFET.
together near the IC.
as possible to the IC)
traces….) should be route as far as possible from the
switching current loops.
+
IN
----- -
12
, high side and Low Side MOSFET, Schottky diode, as
Vin Vout
Vin
I
IN
L
= 2 × V
2
–
2
OUT
Suggestion
, that is, 50% duty cycle). The input,