Datasheet

FB
SW
L1
C
OUT
R
FB2
R
FB1
LM25011A
BST
C
BST
D1
CS
V
OUT
SGND
CSG
R
S
R1
Cff
FB
SW
L1
C
OUT
R
FB2
R
FB1
LM25011A
BST
C
BST
D1
CS
V
OUT
SGND
CSG
R
S
R1
R1 =
V
RIPPLE
x (R
FB2
+ R
FB1
)
'I x R
FB1
LM25011, LM25011-Q1
SNVS617G APRIL 2009REVISED FEBRUARY 2013
www.ti.com
OUTPUT RIPPLE CONTROL (LM25011A)
The LM25011A most likely will require more ripple voltage than is generated across the RS resistor. Additional
ripple can be supplied to the FB pin, in phase with the switching waveform at the SW pin, for proper operation.
The required ripple can be supplied from ripple generated at V
OUT
, through the feedback resistors, as described
in Option A below. Options B and C provide for lower output ripple with one or two additional components.
The amount of additional ripple voltage needed at the FB pin is typically in the range of 30-150mV. Higher
switching frequencies or higher inductor values (less ripple current) require more ripple voltage injected on FB.
Insufficient ripple voltage will result in frequency jitter. For a particular application, add only as much ripple as
needed to stabilize the switching frequency over the required input voltage.
Option A) Lowest Cost Configuration: In this configuration R1 is installed in series with the output capacitor
(C
OUT
) as shown in Figure 24. The inductor’s ripple current passes through R1, generating a ripple voltage at
V
OUT
. The minimum value for R1 is:
(23)
where ΔI is the minimum ripple current amplitude, which occurs at minimum Vin.
Figure 24. Option A – Lowest Cost Ripple Configuration
Option B) Intermediate Ripple Configuration: This configuration generates less ripple at V
OUT
than Option A
above by the addition of one capacitor (Cff), as shown in Figure 25.
Figure 25. Option B – Intermediate Ripple Configuration
Since the output ripple is passed by Cff to the FB pin with little or no attenuation, R1’s value can be chosen so
the minimum ripple at V
OUT
is approximately 150 mVp-p. The minimum value for R1 is calculated from:
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