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參數(shù)資料
| 型號： | SC486EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Complete DDR1/2/3 Memory Power Supply |
| 中文描述： | 完全DDR1/2/3內(nèi)存供電 |
| 文件頁數(shù)： | 11/26頁 |
| 文件大?。?/td> | 382K |
| 代理商： | SC486EVB |
11
2006 Semtech Corp.
www.semtech.com
SC486
POWER MANAGEMENT
EN/PSV: Enable, PSAVE and Soft Discharge (Cont.)
If the EN/PSV pin is pulled low, all three outputs will be
shut down and discharged using switches with a nominal
resistance of 22 Ohms, regardless of the state of the
VTTEN pin. This will ensure that the outputs will be in a
defined state next time they are enabled and also
ensure, since this is a soft discharge, that there are no
dangerous negative voltage excursions to be concerned
about. In order for the soft discharge circuitry to
function correctly, the chip supply must be present.
VTTEN
The VTTEN pin is used to enable the VTT regulator only.
Pulling it high enables the regulator as long as VDDQ/
REF are present. Pulling VTTEN low while EN/PSV is
floating or high will turn off the VTT regulator and leave it
in a high-impedance state for S3 mode (VDDQ and REF
present, VTT high-Z).
VDDQ Output Voltage Selection and Output Sense
The output voltage is set by the feedback resistors R5 &
R9 of Figure 2 below. The internal reference is 1.5V, so
the voltage at the feedback pin will match the 1.5V
reference. Therefore the output can be set to a
minimum of 1.5V. The equation for setting the output
voltage is:
5
8
R
5
R
1
VOUT
+
=
VDDQS is used to sense the output voltages for the on-
time one-shot, t
, and also to generate REF, which is 1/
2 of VDDQ. An RC filter consisting of 10
and 1μF from
VDDQ to VSSA is required (R4 and C2 in Figure 2) to filter
switching frequency ripple.
VDDQ Current Limit Circuit
Current limiting of the SC486 can be accomplished in
two ways. The on-state resistance of the low-side
MOSFETs can be used as the current sensing element or
sense resistors in series with the low-side sources can
be used if greater accuracy is desired. R
sensing is more efficient and less expensive. In both
cases, the R
resistors between the ILIM pin and LX pin
set the over current threshold. This resistor R
is
connected to a 10μA current source within the SC486
which is turned on when the low side MOSFET turns on.
When the voltage drop across the sense resistor or low
side MOSFET equals the voltage across the RILIM
resistor, positive current limit will activate. The high side
MOSFET will not be turned on until the voltage drop across
the sense element (resistor or MOSFET) falls below the
voltage across the R
resistor. In an extreme over-
current situation, the top MOSFET will never turn back
on and eventually the part will latch off due to output
undervoltage (see Output Undervoltage Protection).
The current sensing circuit actually regulates the
inductor valley current (see Figure 3). This means that if
the current limit is set to 10A, the peak current through
the inductor would be 10A plus the peak ripple current,
and the average current through the inductor would be
10A plus 1/2 the peak-to-peak ripple current. The
equations for setting the valley current and calculating
the average current through the inductor are shown
overleaf.
Figure 2
+
C10
VDDQ
C12
1uF
C11
20uF
VTT
C9
1uF
R2
10R
5VRUN
5VSUS
C3
no-pop
R7
10R
REF
R6
10R
C6
1uF
R4
10R
C2
1uF
R5
R9
VDDQ
PGOOD
VBAT
C8
1nF
C1
no-pop
C7
no-pop
R8
0R
PGND1
18
REF
8
EN/PSV
1
TON
2
VDDQS
3
VCCA
5
FB
6
PGD
7
VSSA
4
PGND2
17
DL
19
VDDP
20
ILIM
21
LX
22
DH
23
BST
24
VTTEN
11
VTT
15
PGND2
16
VTTIN
VTTIN
12
13
VTT
14
VTTS
10
COMP
9
U1
SC486
VDDQ
4
1
2
3
5
6
7
8
Q1
R10
C13
1uF
C4
0.1uF
D1
R3
470k
5VSUS
C5
10uF
R1
VBAT
L1
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