sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 10

• Manufacturer Part Number: sc1486aitstr
• Description: Complete Ddr1/2/3 Power Supply Controller
• Manufacturer: Semtech Corporation

OUT1 Output Voltage Selection
The output voltage is set by the feedback resistors R10
& R13 of Figure 2 below. The internal reference is 1.5V,
so the voltage at the feedback pin is multiplied by three
to match the 1.5V reference. Therefore the
output can be set to a minimum of 0.5V. The equation
for setting the output voltage is:
1.8V, 10A
Current Limit Circuit
Current limiting of the SC1486A 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 10
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 resisor,
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
POWER MANAGEMENT
Application Information (Cont.)
VOUT
2006 Semtech Corp.
C12
0u1
0402
10u/25V
1210
C4
+
10u/25V
C13
330u/25m
7343
R7
0R
0402
Figure 2: Setting VDDQ Output Voltage
1210
C3
1
0u1/25V
PGOOD1
+
0603
C14
330u/25m
7343
R
R
ILIM
C2
L1
10
13
R14 470k
2n2/50V
2u4
resistors between the ILIM pin and LX pin
0402
0402
PWR_SRC
C1
5VSUS
µ
0
A current source within the SC1486A
5 .
8
7
6
5
8
7
6
5
IRF7811AV
FDS6676S
D
D
D
D
D
D
D
D
ILIM
S
S
S
G
S
S
S
G
Q2
Q3
resistor. In an extreme over-
1
2
3
4
1
2
3
4
1.8V
VSSA1
C10 0u1
C17
27p
0402
R5
0603
0402
13k3
45k3
0402
17k4
0402
R3
1M
0402
R10
R13
VCCA1
TON1
D1
SOD
323
C20
1n
0402
R1
10R
0402
C8
1u
0603
PGD1
VCCA1
VCCA1
C21
1u
0603
5VSUS
27
22
24
26
23
25
28
3
1
6
7
5
4
2
U1
VDDP1
PGND1
DH1
BST1
LX1
ILIM1
DL1
PGD1
EN/PSV1
VOUT1
FB1
TON1
VCCA1
VSSA1
SC1486A
REFOUT
VDDP2
PGND2
VCCA2
VSSA2
REFIN
ILIM
TON2
DS(ON)
BST2
ILIM2
PGD2
FB2
DH2
DL2
LX2
17
15
20
21
19
18
16
13
12
8
10
9
11
14
is
10
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
below:
The equation for the current limit threshold is as follows:
Where (referring to Figure 2) R
R
For resistor sensing, a sense resistor is placed between
the source of Q3 and PGND. The current through the
source sense resistor develops a voltage that opposes
the voltage developed across R
developed across the R
drop across R
high side MOSFET will not be allowed to turn on. When
using an external sense resistor R
of the sense resistor.
The current limit circuitry also protects against negative
over-current (i.e. when the current is flowing from the
load to PGND through the inductor and bottom MOSFET).
In this case, when the bottom MOSFET is turned on, the
phase node, LX, will be higher than PGND initially. The
SC1486A monitors the voltage at LX, and if it is greater
than a set threshold voltage of 140mV (nom.) the
bottom MOSFET is turned off. The device then waits for
approximately 2µs and then DL goes high for 300ns (typ.)
once more to sense the current. This repeats until
either the over-current condition goes away or the part
I
LIMIT
DS(ON)
10e
of Q3.
6 -
Figure 3: Valley Current Limiting
Valley Current-Limit Threshold Point
R
ILIM
R
SENSE
, a positive over-current exists and the
ILIM
A
SENSE
TIME
resistor reaches the voltage
ILIM
ILIM
is R5 and R
SENSE
. When the voltage
SC1486A
is the resistance
www.semtech.com
SENSE
I
I
I
PEAK
LOAD
LIMIT
is the