IBOUNDARY =

(VIN ± VOUT) x D

2 x L x fSW

LM20146

SNVS563C – FEBRUARY 2008 – REVISED APRIL 2013

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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 VIN 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.

12

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