Datasheet

LM5642, LM5642X
SNVS219K –JUNE 2003–REVISED APRIL 2013 www.ti.com
THERMAL SHUTDOWN
The LM5642 IC will enter thermal shutdown if the die temperature exceeds 160°C. The top and bottom FETs of
both channels will be turned off immediately. In addition, both soft start capacitors will begin to discharge through
separate 5.5 µA current sinks. The voltage on both capacitors will settle to approximately 1.1V, where it will
remain until the thermal shutdown condition has cleared. The IC will return to normal operating mode when the
die temperature has fallen to below 146°C. At this point the two soft start capacitors will begin to charge with
their normal 2.4 µA current sources. This allows a controlled return to normal operation, similar to the soft start
during turn-on. If the thermal shutdown condition clears before the voltage on the soft start capacitors has fallen
to 1.1V, the capacitors will first be discharged to 1.1V, and then immediately begin charging back up.
OUTPUT CAPACITOR DISCHARGE
Each channel has an embedded 480Ω MOSFET 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
If an output over voltage event occurs, the HDRVx will be turned off and LDRVx will be turned on immediately to
discharge the output capacitors of both channels through the inductors.
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 produce excessive electromagnetic interference (EMI), and can
also cause problems in device performance. Therefore, it must be suppressed using one of the following
methods.
When using resistor based current sensing, it is strongly recommended to add R-C filters to the current sense
amplifier inputs as shown in Figure 29. 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.
As shown in Figure 28, adding a resistor in series with the HDRVx pin will slow down the gate drive, thus slowing
the rise and fall time of the top FET, yielding a longer drain current transition time.
Usually a 3.3Ω to 4.7Ω resistor 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 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 increase both rise
and fall times.
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