SEMTECH SC1104A

SC1104A/B
Simple, Synchronous
Voltage Mode PWM Controller
PRELIMINARY
POWER MANAGEMENT
Description
Features
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The SC1104A/B is a versatile voltage-mode PWM controller designed for use in single ended DC/DC power
supply applications. A simple, fixed frequency high efficiency buck regulator can be implemented using the
SC1104A/B with a minimum of external components.
Internal level shift and drive circuitry eliminates the need
for an expensive P-channel, high-side switch. The small
device footprint allows for compact circuit design.
SC1104A/B features include temperature compensated
voltage reference, triangle wave oscillator, current limit
comparator and an externally compensated error
amplifier. Current limit is implemented by sensing the
voltage drop across the top FET’s RDS(ON)..
+5V or +12V operation
High efficiency (>90%)
1% Reference voltage accuracy
Hiccup mode over current protection
Robust output drive
RDS(ON) Current sensing
Industrial temperature range
SO-8 package
Applications
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The SC1104A/B operates at a fixed frequency of 300/
600 kHz, providing an optimum compromise between
efficiency, external component size, and cost. 600kHz
switching frequency is typically reserved for the 5V
operation only.
Termination supplies
Low cost microprocessor supplies
Peripheral card supplies
Industrial power supplies
High density DC/DC conversion
SC1104A/B has a thermal protection circuit, which is
activated if the junction temperature exceeds 150°C.
Typical Application Circuit
Typical Distributed Power Supply
R2
10
C1
220-680pF
R1
2k
+
C3
0.1
C4
10.0
D1
MBRA130L
C5
47/16V
C6
47/16V
C7
47/16V
U1
SC1104A/B
C2
0.01
Vin 5 to 12V
_
R4
1
COMP/SS
2
GND
SENSE
8
VCC
7
3
DL
PHASE
6
4
DH
BST
5
2.32k
R3
1.00k
R5
200-1k
C8
0.1-0.22
R8
10
R6
1-5.1
Q1
Si4884DY
C9
0.1
L1
1.5-3uH
Q2
Si4874DY
R7
1-5.1
+
D2
(opt)
C10
220/4V
C11
220/4V
C12
220/4V
C13
220/4V
C14-17
1.0
3.3V
_
Figure 1
Revision 3, October 2001
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Absolute Maximum Ratings
Parameter
Symbol
Maximum
Units
VCC to GND
15
V
BST to PHASE
15
V
PHASE to GND
-1 to 15
V
DH to PHASE
15
V
DL to GND
15
V
COMP/SS to GND
7
V
SENSE to GND
7
V
Thermal Resistance Junction to Case
θJC
40
°C/W
Thermal Resistance Junction to Ambient
θJ A
160
°C/W
Operating Temperature Range
TA
-40 to +85
°C
Storage Temperature Range
TSTG
-65 to +150
°C
Lead Temperature (Soldering) 10 Sec.
TLead
300
°C
Electrical Characteristics
Unless specified: A: VCC = 12 ± 0.6V, VBST = 23 ± 1V, VOUT = 3.3V, TA = 25oC.B: VCC = 5 ± 0.25V, VBST = 12 ± 0.6V, VOUT = 2.0V, TA = 25oC
Parameter
Symbol
Conditions
Min
Typ
Max
V CC
FSW = 300kHz (nom.), (SC1104A)
4.2
12.6
V CC
FSW = 600kHz (nom.), (SC1104B)
4.2
7
ICC
VCOMP ≤ 0.4V
Units
Pow er Supply
Supply Voltage
V
Supply Current
11
14
mA
Error Amplifier
E/A Transconductance
gm
10
Input Bias Current
IFB
1
Output Sink Current
ISIK
VSENSE ≥ 1.1V; VCOMP = 1.5V
0.7
Source Current
ISC
VSENSE ≥ 0.9V; VCOMP = 1.5V
1.1
FOSC
VCC = 12V ± 0.6V
270
300
330
VCC = 5V ± 0.25V
540
600
660
mS
3
µA
mA
Oscillator
Switching Frequency
Ramp Peak Voltage
© 2001 Semtech Corp.
VP-K
4.75V ≤ VCC ≤ 12.6V
2
2.0
kHz
V
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Electrical Characteristics
Unless specified: A: VCC = 12 ± 0.6V, VBST = 23 ± 1V, VOUT = 3.3V, TA = 25oC.B: VCC = 5 ± 0.25V, VBST = 12 ± 0.6V, VOUT = 2.0V, TA = 25oC
Parameter
Symbol
Conditions
Min
Typ
Max
Units
Ramp Valley Voltage
VV
4.75V ≤ VCC ≤ 12.6V
1.0
V
Maximum Duty Cycle
dcMAX
VCC = 12V (300kHz)
95
%
VCC = 5V (600kHz)
90
MOSFET Drivers
DH Sink/Source Current
SC1104A
IDH
d.c. < 2%, tPW < 100µs
VGS = 4.5V (src)
0.7
DL Sink/Source Current
SC1104A
IDL
VGS = 2.5V (snk)
0.7
DH Sink/Source Current
SC1104B
IDH
d.c. < 2%, tPW < 100µs
VGS = 4.5V (src)
0.5
DL Sink/Source Current
SC1104B
IDL
VGS = 2.5V (snk)
0.5
DH Rise/Fall Time
tr, tf
CL = 3000pF, See Fig. 2
50
DL Rise/Fall Time
tr, tf
CL = 4000pF, See Fig. 2
50
tdt
S ee F i g. 2
80
tOFF
4.75V ≤ Vcc ≤ 12.6V
160
VREF
4.75V ≤ Vcc ≤ 12.6V
0.990
-40 < TA < 85°C
-1
Vtrp
4.75V < Vcc < 12.6V
Vtrp = Vcc - VPHASE
180
SS Source Current
ISRC
VCOMP < 2.5V
1.5
SS Sink Current
ISNK
VCOMP > 0.5V
1.5
Dead Time
DH Minimum Off Time
A
A
ns
ns
Reference Section
Reference Voltage
Reference Accuracy
1.000
1.010
V
1
%
220
mV
Current Limit
Trip Voltage
200
Soft-Start/Enable
Enable Input Threshold
µA
1
Enable Input Current
VCOMP = 0.8V
V
2
mA
4.5
V
Under Voltage Lockout
UVLO Threshold
Vth
-40< TA < 85°C
3.9
4.15
Thermal Shutdow n
Over Temperature Trip Point
TOTP
150
°C
Note: 1. This device is ESD sensitive. Use of standard ESD handling precautions is required.
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2001 Semtech Corp.
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Electrical Characteristics
Figure 2
Block Diagram
Figure 3
© 2001 Semtech Corp.
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Pin Configuration
Ordering Information
Device
Top View
(2)
SC1104XISTR (1)
P ackag e
Temp Range (TJ)
SOIC-8
-40° to 125°C
Note:
(1) In place of “X”: A = 300kHz; B = 600kHz.
(2) Only available in tape and reel packaging. A reel
contains 2500 devices.
(8-Pin SOIC)
Pin Descriptions
Pin
#
Pin Name
1
COMP/SS
2
GND
3
DL
Low side driver output
4
DH
High side driver output
5
BST
Bootstrap, high side driver.
6
PHASE
7
VC C
8
SENSE
©
2001 Semtech Corp.
Pin Function
Error amplifier output. Compensation, soft start/enable.
Ground.
Input from the phase node between the MOSFETs.
Chip bias supply voltage.
Output voltage sense input.
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Theory of Operation
The circuit will be in steady state when Vout =3.3V ,
Vsense = 1V, Icomp = 0 . The COMP voltage and duty
cycle depend on Vin.
Synchronous Buck Converter
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier. The
inverting input of the error amplifier receives its voltage
from the SENSE pin.
Under Voltage Lockout
The under voltage lockout circuit of the SC1104A/B assures that both high-side and low-side MOSFET driver
outputs remain in the off state whenever the supply voltage drops below set parameters. Lockout occurs if VCC
falls below 4.2V typ.
The non-inverting input of the error amplifier is connected
to an internal 1V reference.
The error amplifier output is connected to the
COMPensation pin. The error amplifier generates a current proportional to (Vsense – 1V), which is the COMP
pin output current (Transconductance ~ 10mS). The
voltage on the COMP pin is the integral of the error amplifier current. The COMP voltage is the non-inverting
input to the PWM comparator and controls the duty cycle
of the MOSFET drivers. The size of capacitor Ccomp controls the stability and transient response of the regulator. The larger the capacitor, the slower the COMP voltage changes, and the slower the duty cycle changes.
RDS(ON) Current Limiting
In case of a short circuit or overload, the high-side (HS)
FET will conduct large currents. To prevent damage, in
this situation, large currents will generate a fault condition and begin a soft start cycle.
While the HS driver is on, the phase voltage is compared
to the Vcc pin voltage. If the phase voltage is 200mV
lower than Vcc, a fault is latched and the soft start cycle
begins.
The inverting input voltage of the PWM comparator is
the triangular output of the oscillator.
The voltages are compared during the middle of the HS
pulse, to prevent transients from affecting the accuracy.
When the oscillator output voltage drops below the COMP
voltage, the comparator output goes high. This pulls DL
low, turning off the low-side FET. After a short delay (“dead
time”), DH is pulled high, turning on the high-side FET.
When the oscillator voltage rises back above the error
amplifier output voltage, the comparator output goes low.
This pulls DH low, turning off the high-side FET, and after
a dead time delay, DL is pulled high, turning on the lowside FET. The dead time delay is determined by a
monostable on the chip.
Soft Start
The soft start (or hiccup) circuitry is activated when a
fault occurs. Faults occur for three reasons:
1) Under voltage (VCC < 4.2V)
2) Over temperature (die temperature > 150°C)
3) Over current in high side FET.
All faults are handled the same way. Both DH and DL are
forced low. The error amplifier is turned off, but a 2µA
current flows into the comp pin (soft start current). The
sink current reduces the Comp voltage down to 0.6V
over a period of a few milliseconds. When Vcomp ~ 0.6V,
the fault is cleared and the DL goes high. Also, the soft
start current changes polarity and begins to increase the
voltage on the Comp capacitor. The DH remains low, because Vcomp is less than the lowest excursion of the
oscillator ramp (1.0V). After a few ms, the Vcomp increases to about 1.0V and the DH will start to switch.
The duty cycle will gradually increase, and Vsns will increase. When Vsns ~ 1.00V, the error amplifier turns on
again. The circuit has now reached its operating point. If
a fault occurs during the soft start, the cycle will begin
again (drivers low, Vcomp decreasing down to 0.6V).
The triangle wave minimum is about 1V, and the maximum is about 2V. Thus, if Vcomp = 0.9V, high side duty
cycle is the minimum (~0%) , but if Vcomp is 2.0V, duty
cycle is at maximum ( ~80%).The internal oscillator uses
an on-chip capacitor and trimmed precision current
sources to set the oscillation frequency to 300/600kHz.
Figure 1 shows a 3.3V output converter. If the Vout <3.3V,
then the SENSE voltage < 1V. In this case the error
amplifier will be sourcing current into the COMP pin so
that COMP voltage and duty cycle will gradually increase.
If Vout > 3.3V, the error amplifier will sink current and
reduce the COMP voltage, so that duty cycle will decrease.
© 2001 Semtech Corp.
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SC1104A/B
PRELIMINARY
POWER MANAGEMENT
Outline Drawing - SO-8
Land Pattern - SO-8
Contact Information
Semtech Corporation
Power Management Products Division
652 Mitchell Rd., Newbury Park, CA 91320
Phone: (805)498-2111 FAX (805)498-3804
©
2001 Semtech Corp.
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