Datasheet

I
BOUNDARY
=
(V
IN
± V
OUT
) x D
2 x L x f
SW
V
ss
V
FB
V
EN
V
PGOOD
V
SW
OVP UVP PRE-BIASED
STARTUP
DISABLE
V
ovp
V
uvp
t
PGDGL
0.6V
t
RESETSS
t
PGDGH
V
OVPHYS
V
UVPHYS
LM21215A
www.ti.com SNOSB87B –MARCH 2011–REVISED MARCH 2013
POWERGOOD FLAG
The PGOOD pin provides the user with a way to monitor the status of the LM21215A-1. In order to use the
PGOOD pin, the application must provide a pull-up resistor to a desired DC voltage (i.e. Vin). PGOOD will
respond to a fault condition by pulling the PGOOD pin low with the open-drain output. PGOOD will pull low on
the following conditions – 1) V
FB
moves above or below the V
OVP
or V
UVP
, respectively 2) The enable pin is
brought below the enable threshold 3) The device enters a pre-biased output condition (V
FB
>V
SS
).
Figure 27 shows the conditions that will cause PGOOD to fall.
Figure 27. PGOOD Conditions
LIGHT LOAD OPERATION
The LM21215A-1 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 device will
enter the diode emulation mode preventing significant negative inductor current. The output current 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 (DCM), 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 500mA, several pulses may be skipped in between switching cycles, effectively
reducing the switching frequency and further improving light-load efficiency.
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