SC1548CSK-1.8TR View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC1548CSK-1.8TR' PDF : 14 Pages View PDF
SC1548
POWER MANAGEMENT
Applications Infomation (Cont.)
Please see Table 1 below for recommended resistor
values for some standard output voltages. All resistors
are 1%, 1/10W.
VOUT (V)
1.5
1.8
2.5
2.8
3.0
3.3
R1 (Ω)
18.7
42.2
97.6
124
140
169
R2 (Ω)
100
100
100
102
102
105
Table 1: Recommended Resistor Values For SC1548
The maximum output voltage that can be obtained from
the adjustable option is determined by the input supply
voltage and the RDS(ON) and gate threshold voltage of the
external MOSFET. Assuming that the MOSFET gate
threshold voltage is sufficiently low for the output
voltage chosen and a worst-case drive voltage of 9V,
VOUT(MAX) is given by:
V = V − I • R OUT (MAX )
PWR (MIN )
OUT (MAX )
DS ( ON )( MAX )
To be most effective, the MOSFET RDS(ON) should not be
selected artificially low. The MOSFET should be
chosen so that at maximum required current, it is almost
fully turned on. If, for example, a supply of 1.5V at 4A is
required from a 3.3V ± 5% rail, the maximum allowable
RDS(ON) would be:
( ) R = DS ( ON )(MAX )
0.95 • 3.3 − 1.5 • 1.025
4
≈ 400 m Ω
To allow for temperature effects 200mΩ would be a
suitable room temperature maximum, allowing a peak
short circuit current of approximately 15A for a short time
before shutdown.
Capacitor Selection
Output Capacitors: low ESR aluminum electrolytic or tan-
talum capacitors are recommended for bulk
capacitance, with ceramic bypass capacitors for
decoupling high frequency transients.
Input Capacitors: placement of low ESR aluminum
electrolytic or tantalum capacitors at the input to the
MOSFET (VPWR) will help to hold up the power supply
during fast load changes, thus improving overall transient
response. The 12V supply should be bypassed with a
0.1µF ceramic capacitor.
Short Circuit Protection
Layout Guidelines
The short circuit protection feature of the SC1548 is
implemented by using the RDS(ON) of the MOSFET. As the
output current increases, the regulation loop maintains
the output voltage by turning the FET on more and more.
Eventually, as the RDS(ON) limit is reached, the MOSFET
will be unable to turn on any further, and the output
voltage will start to fall. When the output voltage falls to
approximately 50% of nominal, the LDO controller is
latched off, setting output voltage to 0V. Toggling the
enable pin or cycling the power will reset the latch.
To prevent false latching due to capacitor inrush currents
or low supply rails, the current limit latch is initially
disabled. It is enabled at a preset time (nominally 5ms)
after both IN and EN rise above their lockout points. If
EN is left floating (using the internal resistor pullup), then
V should come up before V , or the device will latch
PWR
IN
off. If the enable function is not being used, EN should
be tied to VPWR.
One of the advantages of using the SC1548 to drive an
external MOSFET is that the bandgap reference and
control circuitry do not need to be located right next to
the power device, thus a very accurate output voltage
can be obtained since heating effects will be minimal.
The 0.1µF bypass capacitor should be located close to
the supply pin, and connected directly to the ground plane.
The ground pin of the device should also be connected
directly to the ground plane. The sense or adjust pin does
not need to be close to the output voltage plane, but
should be routed to avoid noisy traces if at all possible.
Power dissipation within the device is practically
negligible, requiring no special consideration during
layout.
2004 Semtech Corp.
13
www.semtech.com
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