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SC1480AEVB Datasheet(PDF) 12 Page - Semtech Corporation

Part # SC1480AEVB
Description  DDR and DDR Memory VTT power Supply Controller
PDF  23 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1480AEVB Datasheet(HTML) 12 Page - Semtech Corporation

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 2003 Semtech Corp.
www.semtech.com
SC1480A
POWER MANAGEMENT
Applications Information (Cont.)
V
RIPPLE_VBAT(MAX) = 21mVP-P and VRIPPLE_VBAT(MIN) = 16mVP-P
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, V
OUT, should be approximately 15mVP-P at minimum
V
BAT, and worst case no smaller than 10mVP-P. If
V
RIPPLE_VBAT(MIN) is less than 15mVP-P the above component
values should be revisited in order to improve this.
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIM
TR,
starting from the actual static maximum, V
OUT_ST_POS, when
a load release occurs:
V
ERR
V
V
DC
OUT
POS
_
ST
_
OUT
+
=
.or our DDR2 VTT example:
V
OUT_ST_POS = 0.918V
V
TOL
V
POSLIM
TR
OUT
TR
•
=
Where TOL
TR is the transient tolerance. .or our DDR2
VTT example:
POSLIM
TR = 1.972V
The minimum output capacitance is calculated as
follows:
()F
V
POSLIM
2
I
I
L
C
2
POS
_
ST
_
OUT
2
TR
2
)
MAX
(
VBAT
_
RIPPLE
OUT
)
MIN
(
OUT
−


+
•
=
This calculation assumes the absolute worst case
condition of a full-load to no load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps my be
calculated by substituting the desired current for the I
OUT
term.
.or our DDR2 VTT example:
C
OUT(MIN) = 295µ..
We will select 220µ., using one 220µ., 15mΩ
capacitor.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
()
RMS
MIN
_
BAT
OUT
OUT
)
MIN
(
BAT
OUT
)
RMS
(
IN
A
V
I
V
V
V
I
•
−
•
=
.or our DDR2 VTT example:
I
IN(RMS) = 0.95ARMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10µ.,
1210 size, 25V ceramic capacitor can handle up to 3A
RMS.
Refer to manufacturer’s data sheets.
.inally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOS.ET
R
DS(ON) at VGS = 4.5V for purposes of this calculation:
A
2
I
I
I
)
MIN
(
VBAT
_
RIPPLE
OUT
VALLEY
−
=
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
()
Ohms
10
10
4
.
1
R
2
.
1
I
R
6
)
ON
(
DS
VALLEY
ILIM
−
•
•
•
•
=
.or our DDR2 VTT example:
I
VALLEY = 2.47A and RILIM = 9.12kΩ
We select the next lowest 1% resistor value: 9.09kΩ
Thermal Considerations
The junction temperature of the device may be calculated
as follows:
C
P
T
T
JA
D
A
J
°
θ
•
+
=
Where:



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