Qdatasheet_Logo
Integrated circuits, Transistor, Semiconductors Search and Datasheet PDF Download Site

SC461 View Datasheet(PDF) - Semtech Corporation

Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
L
D x VIN
+-
DCR
RL
VL
CL
CC
R1
FB
pin
R2
Figure 18 — FB Voltage by CL Voltage
The magnitude of the FB ripple contribution due to CL is
shown by the following equation.
VFBc L
Vc
L
u
R1
R1 //
// R2 u S
R2 u S u
u CC
CC 
1
Figure 19 shows the equivalent circuit for calculating the
magnitude of the ripple contribution due to the output
voltage ripple.
L DCR
RL VL
CC
CL
FB
pin
VOUT
VOUT
R1
CC
COUT
R2
FB
pin
R1
COUT
R2
Figure 19 — FB Voltage by Output Voltage
The magnitude of the FB ripple contribution due to output
voltage ripple is shown by the following equation.
9)%'9287
'9287
u
5
5 
6u &&
 5
The purpose of this network is to couple the inductor
current ripple information into the feedback voltage such
that the feedback voltage has 90 degrees phase lag to the
switching node similar to the case of using standard high
ESR capacitors. This is illustrated in Figure 20.
VOUT
FB contribution by
output voltage ripple
LX
FB contribution
by CL
Combined FB
IL
Figure 20 — FB voltage in Phaser Diagram
The magnitude of the feedback ripple voltage, which is
dominated by the contribution from CL , is controlled by
the value of R1, R2 and CC . If the corner frequency of (R1//
R2) x CC is too high, the ripple magnitude at the FB pin will
be smaller, which can lead to double-pulsing. Conversely,
if the corner frequency of (R1// R2) x CC is too low, the ripple
magnitude at FB pin will be higher. Since the SC461 regu-
lates to the valley of the ripple voltage at the FB pin, a high
ripple magnitude is undesirable as it significantly impacts
the output voltage regulation. As a result, it is desirable to
select a corner frequency for (R1// R2) x CC to achieve
enough, but not excessive, ripple magnitude and phase
margin. The component values for R1, R2, and CC should
be calculated using the following procedure.
Select CL (typical 10nF) and RL to match with L and DCR
time constant using the following equation.
5/
/
'&5 u &/
Select CC by using the following equation.
&&
|

5  5
u

 u S u IVZ
The resistor values (R1 and R2) in the voltage divider circuit
set the VOUT for the switcher. The typical value for CC is
from 10pF to 1nF.
Dropout Performance
The output voltage adjust range for continuous-conduc-
tion operation is limited by the fixed 250ns (typical)
25
Share Link: GO URL

All Rights Reserved © qdatasheet.com  [ Privacy Policy ] [ Contact Us ]