SC488ML View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC488ML' PDF : 24 Pages View PDF
SC488
POWER MANAGEMENT
Application Information (Cont.)
When using external feedback, and with VDDQ greater
than 1.5V, a small capacitor, CTOP, can be used in parallel
with the top feedback resistor, RTOP, in order to ensure that
ripple at VFB is large enough. C should not be greater
TOP
than 100pF. The value of CTOP can be calculated as fol-
lows, where RBOT is the bottom feedback resistor. Firstly
calculating the value of ZTOP required:
The minimum output capacitance is calculated as fol-
lows:
Iinit
and,
IOUT (MAX )
IRIPPLE_VBAT (MAX )
A
2
Z TOP
RBOT x
0.015
V RIPPLE_VBAT (MIN) 0.015 Ohms
Secondly calculating the value of CTOP required to achieve
this:
C TOP
¨§ 1
¨© ZTOP
1
RTOP
¸·
¸¹
F
2 x Ư x f SW _VBAT (MIN)
Since our example uses internal feedback ,this method
cannot be used, however the voltage seen at the internal
FB point is already greater than 15mV.
C OUT(MIN)
Lx
Iinit 2 Ifinal 2
POSLIMTR2 VOUT_ST_POS2
F
This calculation assumes the condition of a full-load to no-
load step transient occurring when the inductor current is
at its highest. The capacitance required for smaller tran-
sient steps my be calculated by substituting the desired
current for the Ifinal term. In this case Ifinal is set for 5A.
For our example:
Next we need to calculate the minimum output capaci-
tance required to ensure that the output voltage does not
exceed the transient maximum limit, POSLIMTR, starting
from the actual static maximum, V , OUT_ST_POS when a load
release occurs:
COUT(MIN) = 392μF.
We will select 440μF, using two 220μF, 12mΩ capacitors
in parallel.
V OUT_ST_POS V OUT ERR DC V
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
For our example:
VOUT_ST_POS = 1.818V,
I IN ( RMS )
V OUT x VBAT (MIN) VOUT
x
IOUT
VBAT _ MIN
ARMS
For our example:
POSLIM TR V OUT x TOL TR V
Where TOLTR is the transient tolerance. For our example:
POSLIMTR = 1.944V,
IIN(RMS) = 4.17ARMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10μF,
1210 size, 25V ceramic capacitor can handle approxi-
mately 3ARMS. Refer to manufacturer’s data sheets and
derate appropriately.
© 2006 Semtech Corp.
16
www.semtech.com
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