SC283 SEMTECH [Semtech Corporation], SC283 Datasheet - Page 12

• Manufacturer Part Number: SC283
• Description: Dual Channel 2.5MHz, 1.8A Synchronous Step-Down Regulator
• Manufacturer: SEMTECH [Semtech Corporation]

© 2010 Semtech Corp.
Applications Information
Detailed Description
The SC283 is a two channel synchronous step-down
converter. Both channels on this device are designed to
operate in fixed-frequency PWM mode at 2.5MHz and
provide the same current capacity of up to 1.8A. The
switching frequency is chosen to minimize the size of the
external inductor and capacitors while maintaining high
efficiency. Both channels of SC283 are independent.
Operation
During normal operation, the PMOS MOSFET is activated
on each rising edge of the internal oscillator. The voltage
feedback loop uses an internal feedback resistor divider.
The period is set by the internal oscillator. The device
has an internal synchronous NMOS rectifier and does
not require a Schottky diode on the LX pin. The device
operates as a buck converter in PWM mode with a fixed
frequency of 2.5MHz.
Programmable Output Voltage
Both channels on SC283 have fifteen pre-determined
output voltage values which can be individually selected
by programming the CTL input pins (see Table 1 — Output
Voltage Settings). Each CTL pin has an active 1 MΩ
internal pulldown resistor. The 1MΩ resistor is switched in
circuit whenever the CTL input voltage is below the input
threshold, or when the part is in undervoltage lockout.
It is recommended to tie all high CTL pins together and
use an external pull-up resistor to VIN 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
programmable output voltages as this would disable the
device.
SC283 is also capable of regulating a different (higher)
output voltage, which is not shown in the Table 1, via an
external resistor divider. There will be a typical 2µA current
flowing into the VOUT pin. The typical schematic for an ad-
justable output voltage option from the standard 1.0V with
CTLx=[0010], is shown in Figure 1. RFB1 and RFB2 are used
to adjust the desired output voltage. If the RFB2 current is
such that the 2µA VOUT pin current can be ignored, then
RFB1 can be found by Equation 1. RFB2 needs to be low
12
enough in value for the current through the resistor chain
to be at least 20µA in order to ignore the VOUT pin current.
where V
CTL pins.
C
performance. The correct value of C
Equation 2.
C
To simplify the design, it is recommended to program the
desired output voltage from a standard 1.0V as shown in
Figure 1 with the correct C
For programming the output voltage from other standard
voltages, R
Equations 1 and 2.
Maximum Power Dissipation
Each channel of SC283 has its own Θ
only one channel is in operation. Since both channels are
within same package, there is about 50% heat which will be
transferred to the adjacent channel. The equivalent total
thermal impedence will be higher when the neighboring
channel is also in operation. To guarantee an operating
junction temperature of less than 125°C, Figure 2 shows
the maximum allowable power loss of each channel. The
curve is based upon the junction temperature of either
channel reaching a maximum of 125°C. Each channel of
SC283 can support up to 1.8A load current. Figures 3a
R
Enable
FF
FB
FF
V
Output Voltage Up from an Output Voltage of 1.0V
IN
1
[
is needed to maintain good transient response
Figure 1 — Typical Schematic for Adjusting the
nF
=
V
]
OUT
OSTD
10µF
=
C
V
IN_
2
OSTD
−
FB1
5 .
is the pre-determined output voltage via the
V
, R
×
OSTD
R
FB2
FB
⋅
1
and C
R
[
k
(
FB
V
Ω
(CTLx=[0010])
VIN
CTL0_
CTL1_
CTL2_
CTL3_
OUT
2
]
(Channel A or B)
⋅
(
SC283
V
FF
−
OUT
need to be adjusted to meet
0
FF
5 .
VOUT_
calculated from Equation 2.
GND
−
LX_
)
2
V
OSTD
R
R
FB1
FB2
FF
)
×
L_
can be found using
JA
C
(
FF
V
www.semtech.com
of 65°C/W when
OSTD
V
R
for CTLx= 0010 (1.0V)
FB
OSTD
1
−
=
C
SC283
(
OUT_
0
V
OUT
5 .
)
−
) 1
V
(1)
(2)
OUT_
×
R
FB
2