Datasheet

LM26420
www.ti.com SNVS579F –FEBRUARY 2009–REVISED MARCH 2013
USING PRECISION ENABLE AND POWER GOOD
The LM26420's precision enable and power good pins address many of the sequencing requirements required in
today's challenging applications. Each output can be controlled independently and have independent power
goods. This allows for a multitude of ways to control each output. Typically, the enables to each output are tied
together to the input voltage and the outputs will ratiometrically ramp up when the input voltage reaches above
UVLO rising threshold. There may be instances where it is desired that the second output (V
OUT2
) does not turn
on until the first output (V
OUT1
) has reached 90% of the desired set-point. This achieved easily with an external
resistor divider attached from V
OUT1
to EN
2
, see Figure 39.
Figure 39. V
OUT1
controlling V
OUT2
with resistor divider.
If it is not desired to have a resistor divider to control V
OUT2
with V
OUT1
, then the PG
1
can be connected to the
EN
2
pin to control V
OUT2
, see Figure 40. R
PG1
is a pull up resistor on the range of 10kΩ to 100kΩ, 50kΩ is the
suggested value. This will turn on V
OUT2
when V
OUT1
is approximately 90% of the programmed output. NOTE,
this will also turn off V
OUT2
when V
OUT1
is outside the +/-10% of the programmed output.
Figure 40. PG
1
controlling V
OUT2
.
Another example might be that the output is not to be turned on until the input voltage reaches 90% of desired
voltage set-point. This verifies that the input supply is stable before turning on the output. Select R
EN1
and R
EN2
such that the the voltage at the EN pin is greater than 1.12V when reaching the 90% desired set-point.
Figure 41. V
IN
controlling V
OUT
The LM26420's power good feature is design with hysterysis in order to insure no false power good flags are
asserted during large transient. Once power good is asserted high, it will not be pulled low until the output
voltage exceeds +/-14% of the setpoint for a during of ~7.5µS (typ.), see Figure 42.
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