lm21215a-1 National Semiconductor Corporation, lm21215a-1 Datasheet - Page 12

• Manufacturer Part Number: lm21215a-1
• Description: 15a High Efficiency Synchronous Buck Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

www.national.com
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 em-
ulation mode preventing significant negative inductor current.
The output current at which this occurs is the critical conduc-
tion boundary and can be calculated by the following equa-
tion:
Several diagrams are shown in
ous conduction mode (CCM), discontinuous conduction
mode (DCM), and the boundary condition.
Figure 4
illustrating continu-
FIGURE 3. PGOOD Conditions
12
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 capaci-
tance 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 effi-
ciency.
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