lm2715sqx-adj National Semiconductor Corporation, lm2715sqx-adj Datasheet - Page 18

• Manufacturer Part Number: lm2715sqx-adj
• Description: Tft Panel Module
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation
HIGH OUTPUT CAPACITOR ESR COMPENSATION
When using an output capacitor with a high ESR value, or
just to improve the overall phase margin of the control loop,
another pole may be introduced to cancel the zero created
by the ESR. This is accomplished by adding another capaci-
tor, C
parallel with the series combination of R
should be placed at the same frequency as f
zero. The equation for this pole follows:
To ensure this equation is valid, and that C
without negatively impacting the effects of R
must be greater than 10f
CHECKING THE DESIGN
The final step is to check the design. This is to ensure a
bandwidth of 1/6 or less of the frequency of the RHP zero.
This is done by calculating the open-loop DC gain, A
this value is known, you can calculate the crossover visually
by placing a −20dB/decade slope at each pole, and a +20dB/
decade slope for each zero. The point at which the gain plot
crosses unity gain, or 0dB, is the crossover frequency. If the
crossover frequency is less than 1/6 the RHP zero, the
phase margin should be high enough for stability. The phase
margin can also be improved by adding C
earlier in the section. The equation for A
with additional equations required for the calculation:
where R
mum input voltage, g
C2
, directly from the compensation pin V
L
is the minimum load resistance, V
(Continued)
mc ) 0.181fs (in V/s)
m
is the error amplifier transconduc-
ZC
.
C
DC
and C
C2
C2
C
is given below
C
IN
as discussed
and C
to ground, in
can be used
Z1
C
is the mini-
, the ESR
. The pole
DC
C
. After
, f
PC2
18
tance found in the Electrical Characteristics table, and R
SON
the Typical Performance Characteristics section.
LAYOUT CONSIDERATIONS
Vcom AND Gamma
If the supply input, AV
buffer is tied directly to the output of the boost DC/DC
converter, a 100nF bypass capacitor should be connected
close to the device between AV
supply is connected to an external source, a larger bypass
capacitor may be required for desired performance depend-
ing on the external supply voltage ripple and noise.
The Gamma buffer and Vcom amplifier input signal traces
should be routed away from the SW pin. Routing these
traces near the SW pin may inject noise into the device and
affect the performance of the amplifier and/or buffer. If resis-
tor dividers are used to drive the inputs of either the Vcom
amplifier or Gamma buffer the ground connections for them
should be made to AGND to minimize noise.
BOOST SWITCHING REGULATOR
The LM2715 uses two ground connections, PGND and
AGND. The feedback, delay, and compensation networks
should be connected directly to their own dedicated analog
ground plane and this ground plane must connect to the
AGND pin, as shown in Figure 3. No other circuits should
connect to this AGND plane. If no analog ground plane is
available then the ground connections of the feedback, de-
lay, and compensation networks must tie directly to the
AGND pin, as show in Figure 4. Connecting these networks
to the PGND plane can inject noise into the system and
effect performance.
The input bypass capacitor C
device and should connect between V
reduce copper trace resistance which effects input voltage
ripple of the IC. For additional input voltage filtering, a 100nF
bypass capacitor can be placed in parallel with C
the V
output capacitor, C
device and should connect between V
copper trace connections for the C
crease the series resistance, which directly effects output
voltage ripple and efficiency. The feedback network, resis-
tors R1 and R2, should be kept close to the FB pin, and
away from the inductor, to minimize copper trace connec-
tions that can inject noise into the system. Trace connections
made to the inductor and schottky diode should be mini-
mized to reduce power dissipation and increase overall effi-
ciency.
The AGND and PGND pins must connect directly to each
other at the device as shown in Figure 3 and Figure 4.
Failure to do so may affect the performance of the LM2715
and limit its output current capability.
is the value chosen from the graph "R
IN
pin, to shunt any high frequency noise to ground. The
OUT
IN
, should also be placed close to the
, of the Vcom amplifier and Gamma
IN
must be placed close to the
IN
and PGND. If the AV
OUT
IN
OUT
and PGND. This will
capacitor can in-
DSON
and PGND. Any
vs. V
IN
, close to
IN
" in
D -
IN