sc286ultrt Semtech Corporation, sc286ultrt Datasheet - Page 14

• Manufacturer Part Number: sc286ultrt
• Description: Dual Channel 4a Synchronous Step-down Regulator
• Manufacturer: Semtech Corporation

Applications Information (continued)
Under this condition, the output voltage will be the product
of I
falls below the current limit holding level, the output will
charge to the upper PSAVE voltage threshold and return to
normal operation. The SC286 is capable of sustaining an
indefinite short circuit without damage. During soft start, if
current limit has occurred before the SS voltage has reached
400mV, the part enters foldback current limit mode.
Foldback current limit mode will be disabled during soft-
start after the SS voltage is higher than 400mV.
Over-Voltage Protection
In the event of a 15% over-voltage on each independent
output, the PWM drive is disabled with the LX pin floating.
Switching does not resume until the output voltage falls
below the nominal V
Programmable Output Voltage
The SC286 has fifteen pre-determined output voltage
values which can be individually selected for each channel
by programming the CTL input pins (see Table 1 — Output
Voltage Settings). Each CTL pin has an active 500kΩ internal
pull-down resistor. The 500kΩ resistor is switched in circuit
whenever the CTL input voltage is below the input thresh-
old, or when the part is in under voltage lockout. It is
recommended to tie all high CTL pins together and use an
external pull-up resistor to AVIN if there is no enable signal
or if the enable input is an open drain/collector signal. The
CTL pins may be driven by a microprocessor to allow
dynamic voltage adjustment for systems that reduce the
supply voltage when entering sleep states. Avoid all zeros
being present on the CTL pins when changing program-
mable output voltages as this would disable the device.
SC286 is also capable of regulating a different (higher)
output voltage, which is not shown in the Table 1, via an
external resistor divider for each channel. There will be a
typical 2μA current flowing into the VOUTA/B pin. The
typical schematic for an adjustable output voltage option
from the standard 1.0V with CTL
Figure 2. RFB1A/B and RFB2A/B are used to adjust the
desired output voltage. If the RFB2A/B current is such that
the 2μA VOUTA/B pin current can be ignored, then RFB1A/
B can be found using the next equation. RFB2A/B needs to
be low enough in value for the current through the resis-
tor chain to be at least 20μA in order to ignore the VOUTA/B
pin current.
CL_HOLD
and the load resistance. When the load presented
OUT
regulation voltage.
XA/B
= [0010], is shown in
where V
CTL pins.
C
mance. The correct value of C
following equation.
To simplify the design, it is recommended to program the
desired output voltage from a standard 1.0V as shown in
Figure 2 with a proper C
programming the output voltage from other standard
voltages, R
to the previous equations.
Maximum Power Dissipation
Each channel of SC286 has its own Θ
only one channel is in operation. Since both channels
are within the same package, there is about 50% of the
heat generated which will be transferred to the adjacent
channel. The equivalent total thermal impedance will be
higher when the neighboring channel is also in operation.
C
V
V
FF
INA
INB
FF
is needed to maintain good transient response perfor-
[
R
22µF
22µF
nF
C
C
INA
INB
FB
Figure 2 — Output Voltage Programming
]
OSTD
1
R
R
PGOODA
100kΩ
PGOODA
100kΩ
2
FB1
Enable A
Enable B
5 .
is the pre-determined output voltage via the
V
, R
OUT
FB2
R
V
R
R
FB
and C
AVINA
AVINB
OSTD
C
C
0.1µF
0.1µF
1
AVINA
AVINB
V
k [
1Ω
1Ω
OSTD
V
FF
]
FF
PVINA
AGNDA
PGOODA
PVINB
AGNDB
PGOODB
CTL0A
CTL1A
CTL2A
CTL3A
CTL0B
CTL1B
CTL2B
CTL3B
OUT
AVINA
AVINB
need to be adjusted to conform
calculated from Equation 2. For
SC286
V
OUT
R
0
FF
FB
5 .
PGNDB
PGNDA
VOUTA
VOUTB
SSB
2
can be found using the
SSA
LXA
LXB
2
V
OSTD
JA
R
10kΩ
R
10kΩ
R
R
FB2A
FB2B
FB1A
FB1B
of 32.5°C/W when
L
L
C
1nF
C
1nF
SS
SS
(
V
for CTLA
for CTLB
C
C
R
R
FFA
FFB
OSTD
FB1A
FB1B
V
x R
(1.0V)
x R
(1.0V)
= (V
= (V
OSTD
FB2A
X
FB2B
X
C
C
OUTA
OUTB
OUTA
= 0010
OUTB
= 0010
0
-1)
-1)
V
V
OUTA
OUTB
5 .
14
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