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TPS43000
SLUS489 OCTOBER 2001
25
www.ti.com
APPLICATION INFORMATION
grounding
A ground plane is highly recommended. The GND pin of the TPS43000 should be close to the source of the
N-channel MOSFET, the input filter capacitor, and the output filter capacitor. The grounded end of the RT
resistor, the feedback divider resistor, and the SYNC/SD, CCS, CCM, PFM, and BUCK pins (when tied to ground
based on the application) form the signal ground and should be connected to the quietest location of the ground
plane (away from switching elements).
MOSFET gate resistors
The TPS43000 includes low-impedance CMOS output drivers for the two external MOSFET switches. The
NDRV output has a nominal pull-up resistance of 6.5
and a nominal pull-down resistance of 2.25 . The PDRV
output has a nominal pull-down resistance of 3.5
and a nominal pull-up resistance of 2.5 . For
high-frequency operation using low gate charge MOSFETs, no gate resistors are required. To reduce
high-frequency ringing at the MOSFET gates, low-value series gate resistors may be added. These should be
non-inductive resistors, with a value of 2
to 10 , depending on the frequency of operation. Lower values
result in better switching times, improving efficiency.
minimizing output ripple and noise spikes
The amount of output ripple is determined primarily by the type of output filter capacitor and how it is connected
in the circuit. In most cases, the ripple is dominated by the ESR (equivalent series resistance) and ESL
(equivalent series inductance) of the capacitor, rather than the actual capacitance value. Low ESR and ESL
capacitors are mandatory in achieving low output ripple. Surface-mount packages greatly reduce the ESL of
the capacitor, minimizing noise spikes. To further minimize high frequency spikes, a surface-mount ceramic
capacitor should be placed in parallel with the main filter capacitor. For best results, a capacitor should be
chosen whose self-resonant frequency is near the frequency of the noise spike. For high switching frequencies,
ceramic capacitors alone may be used, reducing size and cost.
For applications where the output ripple must be extremely low, a small LC filter may be added to the output.
The resonant frequency should be below the selected switching frequency, but above that of any dynamic loads.
The filter’s resonant frequency is given by:
f
RES +
1
2
p
(L
C)
1 2
where f is the frequency in Hz, L is the filter inductor value in Henries, and C is the filter capacitor value in Farads.
It is important to select an inductor rated for the maximum load current and with minimal resistance to reduce
losses. The capacitor should be a low-impedance type, such as a tantalum.
If an LC ripple filter is used, the feedback point can be taken before or after the filter, as long as the filter’s
resonant frequency is well above the loop crossover frequency. Otherwise, the additional phase lag makes the
loop unstable. The only advantage to connecting the feedback after the filter is that any small voltage drop
across the filter inductor is corrected for in the loop, providing the best possible voltage regulation. However,
the resistance of the inductor is usually low enough that the voltage drop is negligible.
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