Page 1
1 www.semtech.com
SC1548
Linear Fet Controller
POWER MANAGEMENT
Revision 1, January 2001
Typical Application Circuit
The SC1548 is a power supply controller designed to
provide a simple single regulated power supply with over
current protection. It is part of Semtechs SmartLDO
family of products. The SC1548 can provide a 1.818V
power supply for the I/O plane or 1.515V for GTL+ / AGP
from either 3.3V or 2.5V. An adjustable option allows
generation and control of any voltage from 1.263V up to
5V.
SC1548 features include tight output voltage regulation,
an enable control and over current protection. Over
current protection is provided by feedback to the sense
pin. If the output drops below 50% of the nominal
output voltage (typical) for greater than 4ms (typical), the
output will be shut down.
The SC1548 is available in a tiny 5-pin SOT-23 surface
mount package.
u Motherboards
u Graphics cards
u Microcontrollers
u Simple power supplies
u ± 2.5% output accuracy over line, load and
temperature
u 1.515V, 1.818V and adjustable output voltage
options available
u Enable control
u Over current protection
u 5-pin SOT-23 package
1.818V OUT
3.3V IN 12V IN
ENABLE
Q1
IRL530N
C4
0.1uF
+
C1
100uF
+
C2
100uF
+
C3
22uF
U1
SC1548CSK-1.8
1
2
3 4
5
SNS
GND
DRV IN
EN
Adjustable Output Voltage Version
2.5V OUT
3.3V IN 12V IN
ENABLE
Q1
IRL530N
C4
0.1uF
+
C1
100uF
+
C2
100uF
+
C3
22uF
R1
97.6
R2
100
U1
SC1548CSK
1
2
3 4
5
ADJ
GND
DRV IN
EN
Fixed Output Voltage Versions
Description Features
Applications
Page 2
2ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
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Absolute Maximum Ratings
Electrical Characteristics
(1)
Unless specified: TA = 25°C, VIN = 12V, V
PWR
= 3.3V, I
OUT
= 0A. Values in bold apply over full operating temperature range.
Page 3
3ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Notes:
(1) This device is ESD sensitive. Use of standard ESD handling precautions is required.
(2) See Application Circuit on page 1.
(3) See Timing Diagram on page 4.
(4) Connected to FET drain.
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Unless specified: TA = 25°C, VIN = 12V, V
PWR
= 3.3V, I
OUT
= 0A. Values in bold apply over full operating temperature range.
Electrical Characteristics (Cont.)
(1)
Page 4
4ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Timing Diagram
Page 5
5ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Pin Descriptions
Top View
SOT-23-5L
Notes:
(1) Where -X.X denotes voltage options. Available
voltages are: 1.515V (-1.5) and 1.818V (-1.8). Leave blank
for adjustable version.
(2) Only available in tape and reel packaging. A reel
contains 3000 units.
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RT.X.X-KSC8451CS5-32-TOS
Pin Configuration Ordering Information
Block Diagram
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Page 6
6ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
0
25
50
75
100
125
150
0 25 50 75 100 125
T
J
(°C)
I
EN
(µA)
VIN = 12V
V
EN
= 0V
0
100
200
300
400
500
600
700
800
900
1000
0 25 50 75 100 125
T
J
(°C)
t
D(ON)
(ns)
VIN = 12V
Enable Pin Current vs.
Junction Temperature
Enable Delay Time vs.
Junction Temperature
Quiescent Current vs.
Junction Temperature
Start Threshold vs.
Junction Temperature
Enable Threshold Voltage
vs. Junction Temperature
Enable Hysteresis vs.
Junction Temperature
Typical Characteristics
(1)
0
100
200
300
400
500
600
700
800
900
1000
0 25 50 75 100 125
T
J
(°C)
I
Q
(µA)
VIN = 12V, VEN = 3.3V
6.0
6.5
7.0
7.5
8.0
8.5
9.0
0 25 50 75 100 125
T
J
(°C)
UVLO (V)
1.80
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
2.25
2.30
0 25 50 75 100 125
T
J
(°C)
V
TH(EN)
(V)
VIN = 12V
V
EN
rising
0
50
100
150
200
250
300
350
400
450
500
0 25 50 75 100 125
T
J
(°C)
V
HYST
(mV)
VIN = 12V
V
EN
falling
Page 7
7ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Typical Characteristics (Cont.)
(1)
0
25
50
75
100
125
150
175
200
0 25 50 75 100 125
T
J
(°C)
t
D(OFF)
(ns)
VIN = 12V
75
80
85
90
95
100
105
110
115
120
125
0 25 50 75 100 125
T
J
(°C)
I
SNS
(µA)
VIN = 12V
V
EN
= 3.3V
V
SNS
= V
O(NOM)
9.00
9.50
10.00
10.50
11.00
11.50
12.00
0 25 50 75 100 125
T
J
(°C)
V
DRV
(V)
VIN = 12V
V
SNS
= 0V
I
DRV
= 0mA
1.790
1.795
1.800
1.805
1.810
1.815
1.820
1.825
1.830
1.835
1.840
1.845
0 25 50 75 100 125
T
J
(°C)
V
O
(V)
VIN = 12V
V
EN
= 3.3V
3.0V ≤ V
PWR
≤ 3.6V
1mA ≤ I
O
≤ 1A
Disable Delay Time vs.
Junction Temperature
Sense Pin Current vs.
Junction Temperature
Drive Output Voltage vs.
Junction Temperature
Output Voltage (SC1548CSK-1.8)
vs. Junction Temperature
OCP Trip Threshold (SC1548CSK-1.8)
vs. Junction Temperature
Power-Up Output Short Circuit Immunity
vs. Junction Temperature
0
0.2
0.4
0.6
0.8
1
1.2
0 25 50 75 100 125
T
J
(°C)
V
TH(OC)
(V)
VIN = 12V
V
EN
= 3.3V
0
1
2
3
4
5
6
7
8
9
10
0 25 50 75 100 125
T
J
(°C)
Power-up Short Circuit Immunity (ms)
VIN = 12V
V
EN
switched from 0V to 3.3V
R
OUT
= 0
Ω
Two representative parts
shown
Page 8
8ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Output Short Circuit Glitch Immunity
vs. Junction Temperature
Drive Pin Rise Time vs.
Junction Temperature
Drive Pin Fall Time vs.
Junction Temperature
SC1548CSK-1.8 Small Signal Gain
and Phase Shift vs. Frequency
Typical Characteristics (Cont.)
(1)
0
1
2
3
4
5
6
7
8
0 25 50 75 100 125
T
J
(°C)
Short Circuit Glitch Immunity (ms)
VIN = 12V
V
EN
= 3.3V
R
OUT
of 0Ω applied to output
Two representative parts shown
0
200
400
600
800
1000
1200
0255075100125
T
J
(°C)
t
r
(µs)
VIN = 12V
V
EN
switched from 0V to 3.3V
Two repr esentative p a r ts shown
SC1548CSK-1.5 Small Signal Gain
and Phase Shift vs. Frequency
SC1548CSK Small Signal Gain
and Phase Shift vs. Frequency
0
100
200
300
400
500
600
700
800
900
1000
0 25 50 75 100 125
T
J
(°C)
t
f
(µs)
VIN = 12V
V
EN
switched from 3.3V to 0V
Two representative parts shown
-80
-60
-40
-20
0
20
40
60
80
1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06
Gain (dB)
f (Hz)
-360
-315
-270
-225
-180
-135
-90
-45
0
Phase (°)
Gain
Phase
I
OUT
= 1.8A
-80
-60
-40
-20
0
20
40
60
80
1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06
f (Hz)
Gain (dB)
-360
-315
-270
-225
-180
-135
-90
-45
0
Phase (°)
Gain
Phase
I
OUT
= 1.8A
-80
-60
-40
-20
0
20
40
60
80
1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06
f (Hz)
Gain (dB)
-360
-315
-270
-225
-180
-135
-90
-45
0
Phase (°)
V
OUT
= 2V
I
OUT
= 1.8A
Gain
Phase
Page 9
9ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Typical Characteristics (Cont.)
(1)
Trace 1: V
OUT
, AC coupled, 50mV/div.
Trace 2: V
DRV
, 2V/div.
Trace M3: load stepping from 1A to 0A
Timebase: 1µs/div
Load Transient Response, Expanded
Trace 1: V
DRV
, 1V/div.
Trace 2: VEN, 2V/div.
Timebase: 100ns/div
t
D(OFF)
» 36ns
Disable Delay Time, t
D(OFF)
Load Transient Response
Trace 1: V
OUT
, AC coupled, 50mV/div.
Trace 2: V
DRV
, 2V/div.
Trace M3: load stepping from 0A to 1A to 0A
Timebase: 10µs/div
Load Transient Response, Expanded
Trace 1: V
OUT
, AC coupled, 50mV/div.
Trace 2: V
DRV
, 2V/div.
Trace M3: load stepping from 0A to 1A
Timebase: 1µs/div
Page 10
10ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Typical Characteristics (Cont.)
(1)
Trace 1: V
DRV
, 1V/div.
Trace 2: VEN, 2V/div.
Timebase: 500µs/div
tr » 1ms
Drive Output Rise Time, t
r
Trace 1: V
DRV
, 5V/div.
Trace 2: VEN, 2V/div.
Timebase: 2ms/div
SC1548 enabled into a short, therefore V
OUT
< V
TH(OC)
immediately the device is enabled. This device shuts
down after 8ms.
Power-up Output Short Circuit Immunity
Drive Output Fall Time, t
f
Trace 1: V
DRV
, 1V/div.
Trace 2: VEN, 2V/div.
Timebase: 100ns/div
tf » 350ns
Enable Delay Time, t
D(ON)
Trace 1: V
DRV
, 1V/div.
Trace 2: VEN, 2V/div.
Timebase: 250ns/div
t
D(ON)
» 550ns
Page 11
11ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Theory Of Operation
The SC1548 linear FET controller provides a simple way
to drive an N-channel MOSFET to produce a tightly
regulated output voltage from an available, higher,
supply voltage. It takes its power from a 12V supply,
drawing typically 2mA while operating.
It contains an internal bandgap reference which is
compared to the output voltage via a resistor divider.
This resistor divider is internal on the fixed output
voltage options, and user selectable on the adjustable
option. Since the drive pin can pull up to a 9V
guaranteed minimum, the device can be used to
regulate a large range of output voltages by careful
selection of the external MOSFET (see component
selection, below).
The SC1548 includes an active high enable control with
an internal pullup resistor. If this pin is pulled low, the
drive pin is pulled low, turning off the N-channel MOSFET.
If the pin is left open or pulled up to 2.5V, 3.3V or 5V,
then the drive pin will be enabled.
Also included is an overcurrent protection circuit that
monitors the output voltage. If the output voltage drops
below 50% of nominal, as would occur during an
overcurrent or short condition, the device will pull the
drive pin low and latch off.
Fixed Output Voltage Options
Please refer to the Application Circuit on Page 1. The
fixed output voltage parts have an internal resistor
divider that draws a nominal 100µA from the output.
The voltage at the common node of the resistor divider
is then compared to the bandgap reference voltage of
1.263V. The drive pin voltage is then adjusted to
maintain the output voltage set by the resistor divider.
Referring to the block diagram on page 5, the nominal
resistor values are:
egatloVtuptuOk(1R W)k(1R W)
V515.125.236.21
V818.155.536.21
Typical Characteristics (Cont.)
(1)
Output Short Circuit Glitch Immunity
Trace 1: V
DRV
, 5V/div.
Trace 2: V
OUT
, 1V/div.
Timebase: 1ms/div
SC1548 enabled, then shorted, therefore
V
OUT
< V
TH(OC)
immediately the short is applied. This
device shuts down after 5ms.
Note:
(1) See Applications Circuit on page 1.
Applications Infomation
Page 12
12ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
It is possible to adjust the output voltage of the fixed
voltage options, by applying an external resistor divider
to the sense pin (please refer to Figure 1 below). Since
the sense pin sinks a nominal 100µA, the resistor
values should be selected to allow 10mA to flow through
the divider. This will ensure that variations in this current
do not adversely affect output voltage regulation. Thus a
target value for R2 (maximum) can be calculated:
Ω≤
mA10
V
2R
)FIXED(OUT
The output voltage can only be adjusted upwards from the fixed
output voltage, and can be calculated using the
following equation:
VoltsA1001R
2R
1R
1VV
)FIXED(OUT)ADJUSTED(OUT
µ•+
+•=
Adjustable Output Voltage Option
The adjustable output voltage option does not have an
internal resistor divider. The adjust pin connects directly
to the inverting input of the error amplifier, and the
output voltage is set using external resistors (please
refer to Figure 2 above). In this case, the adjust pin
sources a nominal 0.5µA, so the resistor values should
be selected to allow 50µA to flow through the divider.
Again, a target value for R2 (maximum) can be
calculated:
Ω
µ≤A50
V263.1
2R
The output voltage can be calculated as follows:
1RA5.0
2R
1R
1263.1V
OUT
•µ−
+•=
Applications Infomation (Cont.)
VOUT
VPWR
12V IN
ENABLE
R1
R2
Q1
C4
0.1uF
+
C3
22uF
+
C2
100uF
+
C1
100uF
U1
SC1548CSK-X.X
1
2
3 4
5
SNS
GND
DRV IN
EN
Figure 1: Adjusting The Output Voltage of Fixed Output Voltage Options
VOUT
VPWR
12V IN
ENABLE
Q1
C4
0.1uF
+
C1
100uF
+
C2
100uF
+
C3
22uF
R1
R2
U1
SC1548CSK
1
2
3 4
5
ADJ
GND
DRV IN
EN
Figure 2: Setting The Output Voltage of the Adjustable Output Voltage Option
Page 13
13ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Please see Table 1 below for recommended resistor
values for some standard output voltages. All resistors
are 1%, 1/10W.
)V(TUOV(1R W)(2R W)
5.17.81001
8.12.24001
5.26.79001
8.2421201
0.3041201
3.3961501
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 R
DS(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,
V
OUT(MAX)
is given by:
)MAX)(ON(DS)MAX(OUT)MIN(PWR)MAX(OUT
RIVV •−=
Short Circuit Protection
The short circuit protection feature of the SC1548 is
implemented by using the R
DS(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 R
DS(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. Power must be
cycled to 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
PWR
should come up before VIN, or the device will latch
off. If the enable function is not being used, EN should
be tied to V
PWR
.
To be most effective, the MOSFET R
DS(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
R
DS(ON)
would be:
()
Ω≈
•−•
= m400
4
025.15.13.395.0
R
)MAX)(ON(DS
To allow for temperature effects 200mW 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 (V
PWR
) 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.
Layout Guidelines
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.
Applications Infomation (Cont.)
Page 14
14ã 2001 Semtech Corp. www.semtech.com
SC1548
POWER MANAGEMENT
Outline Drawing - SOT-23-5
Semtech Corporation
Power Management Products Division
652 Mitchell Rd., Newbury Park, CA 91320
Phone: (805)498-2111 FAX (805)498-3804
Contact Information
Land Pattern - SOT-23-5
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