SC486 Semtech Corporation, SC486 Datasheet - Page 11

• Manufacturer Part Number: SC486
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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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:
VDDQS is used to sense the output voltages for the on-
time one-shot, t
2 of VDDQ. An RC filter consisting of 10
VDDQ to VSSA is required (R4 and C2 in Figure 2) to filter
switching frequency ripple.
POWER MANAGEMENT
VDDQ
VOUT
2006 Semtech Corp.
REF
VDDQ
C1
no-pop
1
VTT
R
R
R5
5
8
ON
R4
R9
, and also to generate REF, which is 1/
10R
1
5 .
C2
1uF
R6
10R
C6
1uF
R7
C7
no-pop
10R
R8
C3
no-pop
0R
C11
20uF
VBAT
R1
C8
1nF
and 1µF from
5VSUS
C12
1uF
R2
10R
C9
1uF
5VRUN
11
10
14
15
12
13
16
17
3
2
6
8
9
5
4
U1
VTTEN
VDDQS
TON
FB
REF
COMP
VTTS
VCCA
VSSA
VTT
VTT
VTTIN
VTTIN
PGND2
PGND2
Figure 2
11
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
set the over current threshold. This resistor R
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
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.
EN/PSV
PGND1
VDDP
SC486
PGD
ILIM
BST
DH
DL
LX
7
1
24
23
21
22
19
20
18
5VSUS
C13
1uF
ILIM
R10
resistors between the ILIM pin and LX pin
D1
C4
0.1uF
R3
ILIM
4
3
8
2
470k
Q1
resistor. In an extreme over-
5
6
1
VBAT
7
C5
10uF
L1
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SC486
+
C10
ILIM
DS(ON)
PGOOD
VDDQ
is