Datasheet

Status
nPGD
LM5060
R4
GND
VDS fault signal
V
IN
LM5060
SNVS628F –OCTOBER 2009–REVISED APRIL 2013 www.ti.com
Figure 31. Circuitry at the nPGD Pin
INPUT BYPASS CAPACITOR
Some input capacitance from the VIN pin to the GND pin may be necessary to filter noise and voltage spikes
from the V
IN
rail. If the current through Q1 in Figure 21 is very large a sudden shutdown of Q1 will cause an
inductive kick across the line input and pc board trace inductance which could damage the LM5060. In order to
protect the VIN pin as well as SENSE, OVP, UVLO, and nPGD pins from harm, a larger bulk capacitor from VIN
to GND may be needed to reduce the amplitude of the voltage spikes. Protection diodes or surge suppressors
may also be used to limit the exposure of the LM5060 pins to voltages below their maximum operating ratings.
THERMAL CONSIDERATIONS
In normal operation the LM5060 dissipates very little power so that thermal design may not be very critical. The
power dissipation is typically the 2 mA input current times the input voltage. If the application is driving a large
capacitive load application, upon shutdown of the LM5060, the load capacitor may partially, or fully, discharge
back through the LM5060 circuitry if no other loads consume the energy of the pre-charged load capacitor. One
application example where energy is dissipated by the LM5060 is a motor drive application with a large capacitor
load. When the LM5060 is turned off, the motor might also turn off such that total residual energy in the load
capacitor is conducted through the OUT pin to ground. The power dissipated within the LM5060 is determined by
the discharge current of 80 mA and the voltage on the load capacitor.
LARGE LOAD CAPACITANCE
Figure 32 shows an application with a large load capacitance C
L
. Assume a worst case turn off scenario where
Vin remains at the same voltage as C
L
and R
L
is a high impedance. The body diode of Q1 will not conduct any
current and all the charge on C
L
is dissipated through the LM5060 internal circuitry. The dotted line in Figure 32
shows the path of this current flow. Initially the power dissipated by the LM5060 is calculated with the formula:
P = I
GATE-FLT
x V
OUT
where
• I
GATE-FLT
is the sink current of the LM5060 gate control (15)
In applications with a high input voltage and very large output capacitance, the discharge current can be limited
by an additional discharge resistor R
O
in series with the OUT pin as shown in Figure 33. This resistor will
influence the current limit threshold, so the value of R
S
will need to be readjusted.
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