SC286 Datasheet, PDF (14/20 Pages) Semtech Corporation – Dual Channel 4A Synchronous Step-Down Regulator

SC286
Applications Information (continued)
Under this condition, the output voltage will be the product
of I_{CL_HOLD} and the load resistance. When the load presented
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_OUT regulation voltage.
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_XA/B = [0010], is shown in
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.
VINA
C_INA
22µF
V_INB
CINB
22µF
R_PGOODA
100kΩ
R_AVINA 1Ω
C_AVINA
0.1µF
RPGOODA
100kΩ
RAVINB 1Ω
C_AVINB
0.1µF
Enable A
Enable B
PVINA
LXA
SC286
AVINA
L
RFB1A
V_OUTA
CFFA
COUTA
AGNDA
PGOODA
PVINB
AVINB
AGNDB
PGOODB
CTL0A
CTL1A
CTL2A
CTL3A
CTL0B
CTL1B
CTL2B
CTL3B
VOUTA _RFB2A
10kΩ
L
LXB
RFB1B
R_FB1A = (V_OUTA-1)
x R_FB2A
for CTLAX = 0010
(1.0V)
V_OUTB
CFFB
COUTB
VOUTB
RFB2B
10kΩ
R_FB1B = (V_OUTB-1)
x R_FB2B
for CTLBX = 0010
(1.0V)
SSA
PGNDA
C_SS
1nF
SSB
PGNDB
C_SS
1nF
Figure 2 — Output Voltage Programming
R_FB1
V_OUT  V_OSTD
V_OSTD
u R_FB2
where V_OSTD is the pre-determined output voltage via the
CTL pins.
C_FF is needed to maintain good transient response perfor-
mance. The correct value of C_FF can be found using the
following equation.
 _{CFF [nF]}
2.5 u
V_OUT  0.5 ²
u ( ^VOSTD )
R_FB1[k:] u V_OUT  V_OSTD V_OSTD  0.5
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_FF calculated from Equation 2. For
programming the output voltage from other standard
voltages, R_FB1, R_FB2 and C_FF need to be adjusted to conform
to the previous equations.
Maximum Power Dissipation
Each channel of SC286 has its own Θ_JA of 32.5°C/W when
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.
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