SC486 Ver la hoja de datos (PDF) - Semtech Corporation
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SC486 Datasheet PDF : 26 Pages
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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:
VOUT
=
1 +
R5
R8
• 1.5
VDDQS is used to sense the output voltages for the on-
time one-shot, tON, and also to generate REF, which is 1/
2 of VDDQ. An RC filter consisting of 10Ω 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. RDS(ON)
sensing is more efficient and less expensive. In both
cases, the RILIM resistors between the ILIM pin and LX pin
set the over current threshold. This resistor RILIM 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 RILIM 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.
VBAT 5VSUS 5VRUN
VDDQ
R4 10R
C2
C1
R5
1uF
no-pop
REF
R6
10R
R9
C6
1uF
VTT
VDDQ
R7 10R
C3
no-pop
R8 0R
C7
no-pop
C11
20uF
R1
R2
10R
C8
C9
1nF
1uF
C12
1uF
U1
11 VTTEN
3 VDDQS
2 TON
6 FB
8 REF
9 COMP
10 VTTS
5 VCCA
4 VSSA
14
15
VTT
VTT
12
13
VTTIN
VTTIN
16
17
PGND2
PGND2
5VSUS
SC486
PGD 7
EN/PSV 1
BST 24
DH 23
R10
ILIM 21
LX 22
DL 19
VDDP 20
C13
PGND1 18
1uF
R3 470k
VBAT
D1
C4
0.1uF
4
3
8
56
C5
10uF
L1
7
2
Q1
1
PGOOD
VDDQ
+ C10
2006 Semtech Corp.
Figure 2
11
www.semtech.com
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