LM20242

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SNVS534D – OCTOBER 2007 – REVISED MAY 2008

PRE-BIAS STARTUP CAPABILITY

The LM20242 is in a pre-biased state when it starts up with an output voltage greater than zero. This often

occurs in many multi-rail applications such as when powering an FPGA, ASIC, or DSP. In these applications the

output can be pre-biased through parasitic conduction paths from one supply rail to another. Even though the

LM20242 is a synchronous converter, it will not pull the output low when a pre-bias condition exists. During start

up the LM20242 will not sink current until the soft-start voltage exceeds the voltage on the FB pin. Since the

device cannot sink current, it protects the load from damage that might otherwise occur if current is conducted

through the parasitic paths of the load.

POWER GOOD AND OVER VOLTAGE FAULT HANDLING

The LM20242 has built in under and over voltage comparators that control the power switches. Whenever there

is an excursion in output voltage above the set OVP threshold, the part will terminate the present on-pulse, turn-

on the low-side FET, and pull the PGOOD pin low. The low-side FET will remain on until either the FB voltage

falls back into regulation or the zero cross detection is triggered which in turn tri-states the FETs. If the output

reaches the UVP threshold the part will continue switching and the PGOOD pin will be deasserted and go low.

Typical values for the PGOOD resistor are on the order of 100 k

Ω or less. To avoid false tripping during transient

glitches the PGOOD pin has 20 µs of built in deglitch time to both rising and falling edges.

UVLO

The LM20242 has an internal under-voltage lockout protection circuit that keeps the device from switching until

the input voltage reaches 3.9V (typical). The UVLO threshold has 200 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

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 170°C, the LM20242 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 LM20242 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 Equation 1:

(1)

Several diagrams are shown in Figure 8 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. 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.

Copyright © 2007–2008, Texas Instruments Incorporated

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