Datasheet
I
BOUNDARY
=
(V
IN
± V
OUT
) x D
2 x L x f
SW
LM20146
SNVS563C –FEBRUARY 2008–REVISED APRIL 2013
www.ti.com
UVLO
The LM20146 has a built-in under-voltage lockout protection circuit that keeps the device from switching until the
input voltage reaches 2.7V (typical). The UVLO threshold has 45 mV of hysteresis that keeps the device from
responding to power-on glitches during start up. If desired the turn-on point of the supply can be changed by
using the precision enable pin and a resistor divider network connected to V
IN
as shown in Figure 33. in the
design guide.
Thermal Protection
Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event that the maximum
junction temperature is exceeded. When activated, typically at 160°C, the LM20146 tri-states the power FETs
and resets soft start. After the junction cools to approximately 150°C, the part starts up using the normal start up
routine. This feature is provided to prevent catastrophic failures from accidental device overheating.
Light Load Operation
The LM20146 offers increased efficiency when operating at light loads. Whenever the load current is reduced to
a point where the peak to peak inductor ripple current is greater than two times the load current, the part will
enter the diode emulation mode preventing significant negative inductor current. The point at which this occurs is
the critical conduction boundary and can be calculated by the following equation:
(1)
Several diagrams are shown in Figure 28 illustrating continuous conduction mode (CCM), discontinuous
conduction mode, and the boundary condition.
It can be seen that in diode emulation mode, whenever the inductor current reaches zero the SW node will
become high impedance. Ringing will occur on this pin as a result of the LC tank circuit formed by the inductor
and the parasitic capacitance at the node. If this ringing is of concern an additional RC snubber circuit can be
added from the switch node to ground.
At very light loads, usually below 100 mA, several pulses may be skipped in between switching cycles, effectively
reducing the switching frequency and further improving light-load efficiency.
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