SC2738 Semtech Corporation, SC2738 Datasheet - Page 8

• Manufacturer Part Number: SC2738
• Description: Ultra Low Output Voltage Dual Linear FET Controller
• Manufacturer: Semtech Corporation

MOSFETs: very low or low threshold N-channel MOSFETs
are required. Selecting FETs rated for V
will depend upon the available drive voltage (6.9V from
12V in or 4.85V from 5V in), the output voltage and
output current. For the device to work under all
operating conditions, a maximum R
ensure that the output will never go into dropout:
Note that R
the minimum V
Setting The Output Voltage: the adjust pins connect
directly to the inverting input of the error amplifiers, and
the output voltage is set using external resistors (please
refer to the Typical Application Circuit on page 1).
Using output 1 as an example, the output voltage can
be calculated as follows:
The input bias current for the adjust pin is so low that it
can be safely ignored. To avoid picking up noise, it is
recommended that the total resistance of the feedback
chain be less than 100k .
Please see Table 1 on this page for recommended
resistor values for some standard output voltages. All
resistors are 1%, 1/10W.
The maximum output voltage that can be obtained from
each output is determined by the input supply voltage
and the R
external MOSFET. Assuming that the MOSFET gate
threshold voltage is sufficiently low for the output
voltage chosen and the worst-case drive voltage, V
is given by:
R
POWER MANAGEMENT
Applications Infomation (Cont.)
V
V
OUT
OUT
DS
2004 Semtech Corp.
(
ON
(
MAX
)(
0
MAX
)
5 .
)
DS(ON)
DS(ON)
V
DRAIN
1
V
GS
IN
must be met at all temperatures and at
R
and gate threshold voltage of the
R
(
condition.
MIN
(
MIN
1
2
I
OUT
)
)
(
V
MAX
I
OUT
OUT
)
(
(
MAX
MAX
)
)
R
DS(ON)
DS
(
GS
ON
must be met to
of 2.7V or 4.5V
)(
MAX
)
OUT(MAX)
8
Design Example
Goal: 1.5V±5% static and ±7% transient @ up to 4A
from 2.5V±5% and 5V±5%
Total window for DC error and ripple is ±75mV.
Total window for transient is ±105mV.
Since this device is linear, and assuming that it has been
designed to not ever enter dropout, we do not have ripple
on the output.
The DC error for this output is the sum of:
V
Feedback chain tolerance = ±1% = ±15mV
Load regulation = ±0.25% = ±3.8mV
Set resistors per Table 1 should be 20.0k
10.0k
Total DC error = ±4.75% = 71.3mV
This leaves ±2.25% = 33.7mV for the load transient ESR
spike, therefore:
Bulk capacitance required is given by:
Where dI is the maximum load current step, t is the
maximum regulator response time and dV is the
allowable voltage droop. Therefore with dI = 4A,
t = 1µs, and dV = 33.7mV:
R
C
Table 1: Recommended Resistor Values For SC2738
REF
ESR
BULK
accuracy = ±3.5% = ±52.5mV
(
MAX
V
(
MIN
O
)
1
U
1
1
2
3
(bottom).
0 .
)
2 .
5 .
5 .
3 .
T
5
33
(
) V
dI
7 .
4
dV
A
mV
t
F
8
R
4 .
1
m
r o
2
4
6
1
1
R
1
0
5
3
4
3 .
0 .
0 .
0 .
4 .
3
www.DataSheet4U.com
k (
)
PRELIMINARY
R
www.semtech.com
SC2738
2
r o
1
1
1
1
1
1
1
R
0
0
0
(top) and
3 .
3 .
0 .
0 .
0 .
4
k (
)