LM2642
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SNVS203I –MAY 2002–REVISED APRIL 2013
POWER GOOD
A power good pin (PGOOD1) is available to monitor the output status of Channel 1. As shown in Figure 24, the
pin connects to the output of an open drain MOSFET, which will remain open while Channel 1 is within operating
range. PGOOD1 will go low (low impedance to ground) under the following four conditions:
1. Channel 1 is turned off
2. Channel 1 output falls below 90.3% of nominal (UVPG1)
3. OVP on either channel
4. UVP on either channel
When on, the PGOOD1 pin is capable of sinking 0.95mA (typical). If an OVP or UVP condition occurs, both
channels will latch off, and the PGOOD1 pin will be latched low. During a UVPG1 condition, however, PGOOD1
will not latch off. The pin will stay low until Channel 1 output voltage returns to 94% (typical) of nominal. See
Vpwrgd in the Electrical Characteristics table.
OUTPUT CAPACITOR DISCHARGE
Each channel has an embedded 480ΩMOSFET with the drain connected to the SWx pin. This MOSFET will
discharge the output capacitor of its channel if its channel is off, or the IC enters a fault state caused by one of
the following conditions:
1. UVP
2. UVLO
3. Thermal shut-down (TSD)
If an output over voltage event occurs, the HDRVx will be turned off and LDRVx will be turned on immediately to
discharge the output capacitor of both channels through the inductor.
BOOTSTRAP DIODE SELECTION
The bootstrap diode and capacitor form a supply that floats above the switch node voltage. VLIN5 powers this
supply, creating approximately 5V (minus the diode drop) which is used to power the high side FET drivers and
driver logic. When selecting a bootstrap diode, Schottky diodes are preferred due to their low forward voltage
drop, but care must be taken for circuits that operate at high ambient temperature. The reverse leakage of some
Schottky diodes can increase by more than 1000x at high temperature, and this leakage path can deplete the
charge on the bootstrap capacitor, starving the driver and logic. Standard PN junction diodes and fast rectifier
diodes can also be used, and these types maintain tighter control over reverse leakage current across
temperature.
SWITCHING NOISE REDUCTION
Power MOSFETs are very fast switching devices. In synchronous rectifier converters, the rapid increase of drain
current in the top FET coupled with parasitic inductance will generate unwanted Ldi/dt noise spikes at the source
node of the FET (SWx node) and also at the VIN node. The magnitude of this noise will increase as the output
current increases. This parasitic spike noise may turn into electromagnetic interference (EMI), and can also
cause problems in device performance. Therefore, it must be suppressed using one of the following methods.
It is strongly recommended to add R-C filters to the current sense amplifier inputs as shown in Figure 26. This
will reduce the susceptibility to switching noise, especially during heavy load transients and short on time
conditions. The filter components should be connected as close as possible to the IC. Note that these filters
should be used when a current sense resistor is used.
As shown in Figure 25, adding a resistor in series with the SWx pin will slow down the gate drive (HDRVx), thus
slowing the rise and fall time of the top FET, yielding a longer drain current transition time.
Usually a 3.3Ωto 4.7Ωresistor is sufficient to suppress the noise. Top FET switching losses will increase with
higher resistance values.
Small resistors (1-5 ohms) can also be placed in series with the HDRVx pin or the CBOOTx pin to effectively
reduce switch node ringing. A CBOOT resistor will slow the rise time of the FET, whereas a resistor at HDRV will
reduce both rise and fall times.
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