1 A Synchronous Switchi ng Regulator
SC1409
June 28, 2000
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
DESCRIPTION
The SC1409 is a versatile, voltage-mode switching
regulator designed for use in battery operated power
supply applications. A simple, fixed-voltage buck regulator can be implemented using the SC1409 with a
minimum of external components. Internal MOSFETs
and control circuitry minim ize board space and design
time. The sm all device footprint allows for compact
circuit design.
SC1409 features include a tem perature compensated
voltage reference, triangle wave oscillator, hiccup
mode current limit, an internally compensated error
amplifier, overcurrent temperature protection, soft
start, and under voltage lock out.
The SC1409 operates at a selectable, fixed frequency
of 300kHz or 500kHz, or can be synchronized to an
external clock within this range, providing an optimum
compromise between efficiency, external component
size, and cost.
FEATURES
•= Internal MOSFETs
•= Excellent efficiency >88%
•= 1% Voltage accuracy
•= Built-in over current protection
•= 1A Output current
•= 5µA Standby current
•= Industrial temperature range
•= 300kHz/500kHz selectable frequency
APPLICATIONS
•= Handheld equipment
•= PDA power supplies
•= Notebook I/O power supplies
•= Peripheral card supplies
•= High density DC/DC conversion
ORDERING INFORMATION
DEVICE
SC1409ISTR SO-8
(1)
PACKAGE
APPLICATION CIRCUIT
Vin=5V
Vout R3 R4
3.45V 1.87k 1.00k
3.30V 1.74k 1.00k
3.10V 1.58k 1.00k
2.90V 1.40k 1.00k
2.80V 1.33k 1.00k
2.50V 1.07k 1.00k
1.50V 249 1.00k
+
C1
47uF/16V
C3
0.033uF
Note:
(1) Only available in tape and reel packaging. A reel
contains 2500 devices.
R1
OPEN
U1
ISETSYNC/FSET
2
EN/SS
3
AGND
4 5
FB PGND
SC1409IS
VIN
PHASE
81
7
6
L1 100uH
C6
0.1uF
R2
R4
Vout
+
C2
47uF/16V
1
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
1 A Synchronous Switchi ng Regulator
SC1409
June 28, 2000
ABSOLUTE MAXIMUM RATINGS
Parameter Symbol Maximum Units
Supply Voltage V
Ground Differential P
to GND ± 1 V
GND
Input Voltage V
Operating Ambient Temperature Range T
Operating Junction Temperature Range T
Storage Temperature T
Lead Temperature (Soldering) 10 seconds T
Thermal Resistance, Junction to Ambient
Thermal Resistance, Junction to Case
θ=JA
θ=JC
CC
IN
A
J
STG
L
-0.3 to +7 V
-0.3 to +7 V
-40 to +85 °C
-40 to +125 °C
-60 to +150 °C
300 °C
165 °C/W
40 °C/W
ESD Rating (Human Body Model) ESD 2 kV
ELECTRICAL CHARACTERISTICS
Unless specified; TA = 25°C, V
Values in bold apply over full operating t emperature range.
= 5V, VO = 1.8V. Output current = 1A. P er typical application circuit .
IN
Parameter Symbol Conditions Min Typ Max Units
Reference V
REF
1.191 1.203 1.215 V
1.179 1.227
Feedback Bias Current I
Quiescent Current I
Shutdown Quiescent Current I
FB
Q
Current into VCC pin 2.0 4.0 mA
Io=0mA
Q
Enable = 0V 5.0 uA
10.0 50.0 nA
15
10
Regulation Load REG
Regulation Line REG
LOAD
LINE
Oscillator Frequency OSC FSET=Vin
Max Duty Cycle
Output Current Limit I
δ
max
O
IO = 100mA to 1A 0.5 1.0 %
0.5 %
FSET=GND
425
255
500
300
575
345
kHz
kHz
90 95 %
1.5 A
Soft start charging current 32 uA
EN/SS Shutdown Threshold 1.0 V
UVLO Trip point Vin Increasing
Vin decreasing
4.5
4.1
V
V
RDS(ON) R
DS(ON)
Top side PFET 250 375
mΩ
525
Bottom side NFET 175 250
mΩ
350
2
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
BLOCK DIAGRAM
1 A Synchronous Switchi ng Regulator
SC1409
EN/SS
AGND
SYNC/FSET
FB
VREF
UV LOCKOUT
THERMAL
SHUTDOWN
SOFT
START
OC_DET
I
2I
OSCILLATOR
ERROR
AMP
+
-
4.5V
-
+
-
+
1.5V
VIN
1:800
S
Q
R
S
Q
R
R
Q
S
R
-
+
Q
S
SHOOT THRU
CONTROL
+
-
60nS DELAY
ISET
PHASE
PGND
PIN CONFIGURATION
Top View
SYNC/FSET
EN/SS
AGND
FB
1
3
4
(8 LEAD SOIC)
8
7
6
5
ISET
VIN2
PHASE
PGND
PIN DESCRIPTION
Pin # Pin Name Pin Function
1 SYNC/FSET Sets frequency, allows synchronization.
FSET = V
FSET = GND, f = 300kHz
2 EN/SS Enable and Soft Start pin.
3 AGND Analog ground.
4 FB Output voltage feedback.
5 PGND Device power ground.
6 PHASE Power output to the inductor.
7 VIN Input voltage.
8 ISET Current limit programming input.
, f = 500kHz
IN
3
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
1 A Synchronous Switchi ng Regulator
SC1409
50
40
30
20
Gain (dB)
10
0
-10
100E+0 10E+3 1E+6 100E+6
Frequency (Hz)
Phase
Gain
Error Amplifier, Gain and Phase
0.4%
Vo=3.3V
0.2%
0.0%
-0.2%
Line Regulation (%)
-0.4%
Io=0.2A
Ta=25
o
C
180
135
90
45
0
-45
-90
0.0%
-0.2%
Phase (deg)
Load Reg (%)
-0.4%
-0.6%
0.0 0.2 0.4 0.6 0.8 1.0
Load Regulation Characteristic
90%
80%
Efficiency (%)
70%
Io (A)
25mOhm
3.3V; f=300kHz
3.3V; f=500kHz
2.5V; f=300kHz
2.5V; f=500kHz
-0.6%
4.4 4.6 4.8 5.0 5.2 5.4 5.6
Vin (V)
Typical Line Regulation
1.0
0.8
0.6
0.4
0.2
Output Voltage - Normali zed ( V )
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
EN/SS Voltage (V)
Soft Start control of Vout
60%
0.00.20.40.60.81.0
Io(A)
Efficiency
Ripple; Vo=3.3V; Io=1A; f=300kHz
4
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
1 A Synchronous Switchi ng Regulator
SC1409
8.0
7.0
6.0
5.0
4.0
3.0
Quiescent Current (mA)
2.0
1.0
0.0
-50 -25 0 25 50 75 100 125
Shutdown Quiescent Current
Quiescent Current
Temperature (
o
C)
Quiescent current vs temperature
0.45
0.40
0.35
0.30
)
Ω
Ω
Ω
Ω
0.25
(
0.20
DS(ON)
R
0.15
0.10
0.05
0.00
-50 -25 0 25 50 75 100 125
P-Channel (Topside)
N-Channel (Bottomside)
Temperature (
o
C)
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
550
500
450
400
350
Frequency (KHz)
Shutdown Quiescent (uA)
300
250
-50 -25 0 25 50 75 100 125
Frequency (FSET=VIN)
Max Duty Cycle
Frequency (FSET=GND)
Temperature (
o
C)
Frequency and max. duty cycle vs temperature
4.5
4.4
4.3
(V)
4.2
IN
V
4.1
4.0
3.9
-50 -25 0 25 50 75 100 125
VIN Increasing
VIN Decreasing
Temperature (
o
C)
100
98
96
94
92
90
88
Duty Cycle (%)
FET R
50
45
40
35
30
25
20
15
Soft Start Current (uA)
10
5
0
-50 -25 0 25 50 75 100 125
vs temperature
DS(ON)
Temperature (
o
C)
Soft start current vs temperature
UVLO trip point vs temperature
1.206
1.204
1.202
1.200
Reference Voltage (V)
1.198
1.196
-50 -25 0 25 50 75 100 125
Temperature (
o
C)
Reference voltage vs temperature
5
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
1 A Synchronous Switchi ng Regulator
SC1409
THEORY OF OPERATION
General
The SC1409 is a synchronous buck controller with
on-board FETs. It uses top FET sensing for current limit
together with a soft-start c ircuit and hiccup over current
protection. The EN/SS pin can be pulled low to put the
SC1409 in a low quiescent shutdown state. It uses a
wide bandwidth error amplifier for optimum transient
response and minimum external compensation requirements. Overcurrent lim it is preset at 1.5A, but can be
set at a lower value by use of an external ISET resistor.
EN/SS
When power is first applied to the device, the EN/SS
pin sources approximately 1uA to the external soft-start
capacitor, most of the rest of the internal circuitry is
disabled and the device is in low-power mode. If the
soft-start pin is held low, the SC1409 will remain in lowpower mode, drawing approximately 5uA from the s upply. Once the soft start pin is released, the external softstart capacitor will charge up slowly to the EN/SS
threshold, where the source current will increase to
approximately 30uA, charging the soft-start capacitor
more quickly. The voltage at the soft- start pin modifies
the internal reference voltage and hence controls the
output voltage (see Soft-Start control of Vout). Once
the soft start capacitor has charged to approximately
3V, the reference voltage at the input to the error amp
will have reached its final value and the output voltage
will no longer be under the control of the soft-start.
Overcurrent Limit
The SC1409 internally mirrors and scales the current
through the top FET. This scaled, mirrored current is
used to detect an overcurrent condition. In the event an
overcurrent is detected, the output is immediately disabled and the soft-start capacitor is discharged at
approximately 30uA until SS/EN reaches 1.5V. At this
point the output is enabled and a normal soft-start cycle
is initiated. If the fault persists, the output will be disabled, the soft-start c apacitor will continue charging at
30uA until SS/EN reaches 4.5V before the discharge
cycle is initiated.
Some combinations of high output voltage, low inductance and high output capacitance will cause this
“hiccup” mode of operation to continue even if the fault
is cleared. In this case, input power will need to be
cycled, or EN/SS cycled through shutdown to restore
normal operation.
Operating Frequency
The frequency of operation can be set to a nominal
300kHz by grounding the FSET pin, or to a nominal
500kHz by returning the FSET pin to VIN. The SC1409
can also be synchronized to an external clock using the
FSET pin.
LAYOUT GUIDELINES
The SC1409 should preferably be laid out over a
ground plane. Since the power section and the control
section of the IC share a common VIN pin, the VIN pin
must be decoupled to ground by a 0.1uF ceramic capacitor as close to the IC as possible. The traces between VIN and the decoupling capacitor and the decoupling capacitor and the PGND pin should be 25
mils wide and no more than 100 mils long. The main
input capacitor should be placed within about 1” of the
IC using solid copper areas to connect to the IC.
Ideally AGND should be returned to the output capacitor and connected to the ground plane at that point.
Feedback resistors and all traces associated with them
should be kept away from the phase node. The inductor should be placed as close to the PHASE pin as
possible to minimize EMI.
These principles are illustrated in the Evaluation Board
shown below.
Evaluation Board Top Copper and Assembly Diagrams
6
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
5V in
1 A Synchronous Switchi ng Regulator
SC1409
U1
2
EN/SS
3
AGND
4 5
+
C1
47uF
GND
C3
0.033uF
FB PGND
SC1409IS
PHASE
Typical Application Circuit
COMPONENT SELECTION
Input Capacitor
The Input capacitor(s) must be capable of 0.5A RMS
ripple current. Total capacitance values of 22uF to
47uF are suitable with Esr of less than 350mΩ. Typical
capacitors would be AVX TPS Tantalum surface
mount type TPSC476M010R350 or Panasonic Specialty Polymer Aluminum Electrolytic surface mount
type ECGC0KB470RA. Other types of capacitor are
also suitable, such as high value ceramic or Aluminum
electrolytics.
Inductor
Suitable Inductor values are in the range of 47uH to
330uH, higher inductor values give lower output voltage ripple and slower transient response time. Inductor
ripple current acts with the output capacitor ESR to
produce output voltage ripple. For a given output voltage ripple requirement, L can be calculated from
()
L
≥
Output Capacitor
The output capacitor requirements are similar to the
input capacitor except ripple current is lower. Similar
capacitors will be suitable.
VESRVV
⋅⋅−
OUTOUTIN
VfV
⋅⋅
INRIPPLE
R1
OPEN
81
ISETSYNC/FSET
7
VIN
6
100uH
Coilcraft
DO3316P-104
C6
0.1uF
R2
1.74k*
R4
1.0k
Place C6 across pins 7 and 5
of SC1409, as close as
possible to package
C4
OPEN
R3
OPEN
C5
OPEN
R5
OPEN
+
C2
47uF
3.3V OutL1
Soft Start Capacitor
The soft start capacitor value determines the soft start
time according to the following equation:-
C = 0.015t
where C is the soft start capacitor value in uF
and t is the soft start time in mS
Startup from a shutdown condition will take longer than
this since the soft start capacitor must charge from its
shutdown value to approximately 1V at a charge current of 1uA before the shutdown is released and true
soft start begins.
Divider Resistors
The current through the divider chain should be at least
1000 times higher than the FB input bias current to
eliminate ratio errors. This gives a maximum value for
the bottom divider resistor (R4) of 24kΩ
The top resistor (R2) can then be determined from
R2 = (Vo-1.203)·R4/1.203
Compensation
Normally, external compensation is not recommended,
however, if required, loop response can be tailored
with suitable values for R3, R5, C4 and C5. Consult the
factory for details.
7
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
June 28, 2000
OUTLINE DRAWING
1 A Synchronous Switchi ng Regulator
JEDEC
REF: MS-012AA
SC1409
LAND PATTERN SO-8
ECN00-1096
8
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
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