lm2711 National Semiconductor Corporation, lm2711 Datasheet - Page 17

• Manufacturer Part Number: lm2711
• Description: Step-up Pwm Dc/dc Converter Integrated With 4 Buffers
• Manufacturer: National Semiconductor Corporation

Operation
performance of the buffers. If the capacitance presented by
the load is less than 5nF external components will be re-
quired as the load itself will not ensure stability. No external
compensation in this case will lead to oscillation of the buffer
and an increase in power consumption. A single 5nF or
greater capacitor on the output will ensure a stable buffer
with no oscillations. For applications requiring a higher slew
rate, a good choice for compensation is to add a 50Ω (Rox)
in series with a 1nF (Cox) capacitor from the output of the
buffer to ground. This allows for driving zero to infinite ca-
pacitance loads with no oscillations, minimal overshoot, and
a higher slew rate than using a large capacitor. The high
phase margin created by the external compensation will
guarantee stability and good performance for all conditions.
For noise sensitive applications greater output capacitance
may be desired. When the power supply for the buffers (Vs+)
is connected to the output of the switching regulator, the
output ripple of the regulator will produce ripple at the output
of the buffers.
LAYOUT CONSIDERATIONS
The LM2711 uses two separate ground connections, GND
for the driver and NMOS power device of the boost regulator
and AGND for the sensitive analog control circuitry of the
boost regulator and the V
AGND and GND pins should be tied directly together at the
(Continued)
COM
and Gamma buffers. The
FIGURE 4. Single Layer Layout
FIGURE 3. Multi-Layer Layout
17
package, see Figure 3 and Figure 4. The feedback, softstart,
and compensation networks should be connected directly to
a dedicated analog ground plane and this ground plane must
connect to the AGND pin, as in Figure 3. If no analog ground
plane is available then the ground connections of the feed-
back, softstart, and compensation networks must tie directly
to the AGND pin, as show in Figure 4. Connecting these
networks to the GND pin can inject noise into the system and
effect performance. For 600kHz operation the FSLCT pin
should be tied to an analog ground plane or directly to the
AGND pin. For 1.25MHz operation the FSLCT pin should be
tied to the V
The input bypass capacitor C
IC. This will 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
noise to ground. The output capacitor, C
placed close to the IC. Any copper trace connections for the
C
directly effects output voltage ripple and efficiency. The feed-
back network, resistors R1 and R2, should be kept close to
the FB pin, and away from the inductor, to minimize copper
trace connections that can inject noise into the system.
Trace connections made to the inductor and schottky diode
should be minimized to reduce power dissipation and in-
crease overall efficiency.
OUT
capacitor can increase the series resistance, which
IN
, close to the V
IN
pin.
IN
20046853
pin, to shunt any high frequency
IN
must be placed close to the
20046852
OUT
, should also be
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