Datasheet

Voltage
at CS
0V
V
RIPPLE
t
OFF
t
ON
Inductor
Current
'I
Current
Limit
Threshold
Normal
Operation
Load
Current
Increases
Current
Limited
'I
Inductor
Current
Decreases
Load Current
Normal
Operation
Voltage at the FB Pin
2.51V
Voltage at the CS Pin
0V
I
PK
I
O1
I
O2
I
OCL
LM25011, LM25011-Q1
SNVS617G –APRIL 2009–REVISED FEBRUARY 2013 www.ti.com
Figure 18. Normal and Current Limit Operation
RIPPLE REQUIREMENTS
The LM25011 requires about 25mVp-p of ripple voltage at the CS pin. Higher switching frequency may require
more ripple. That ripple voltage is generated by the decreasing recirculating current (the inductor’s ripple current)
through R
S
during the off-time. See Figure 19.
Figure 19. CS Pin Waveform
The ripple voltage is equal to:
V
RIPPLE
= ΔI x R
S
(7)
where ΔI is the inductor current ripple amplitude, and R
S
is the current sense resistor at the CS pin.
More ripple can be achieved by decreasing the inductor value.
The LM25011A, with its shorter minimum off-time, typically will require more ripple than the LM25011. An
external circuit to increase the effective ripple voltage may be needed. Different methods of generating this ripple
are explained in the “Application Information” section.
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