LT1533
Ultralow Noise
1A Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1533 is a new class of switching regulator designed
to reduce conducted and radiated electromagnetic interfer-
ence (EMI). Ultralow noise and EMI are achieved by providing
user control of the output switch slew rates. Voltage and
current slew rates can be independently programmed to
optimize switcher harmonic content versus efficiency. The
LT1533 can reduce high frequency harmonic power by as
much as 40dB with only minor losses in efficiency.
The LT1533 utilizes a dual output switch current mode
architecture optimized for low noise topologies. The IC
includes two 1A power switches along with all necessary
oscillator, control and protection circuitry. Unique error amp
circuitry can regulate both positive and negative voltages.
The internal oscillator may be synchronized to an external
clock for more accurate placement of switching harmonics.
Protection features include cycle by cycle current limit pro-
tection, undervoltage lockout and thermal shutdown.
Low minimum supply voltage and low supply current during
shutdown make the LT1533 well suited for portable applica-
tions. The part may also be forced into a 50% duty cycle mode
for unregulated applications. The LT1533 is available in the
16-pin narrow SO package.
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Greatly Reduced Conducted and Radiated EMI
(<100
µ
V
P-P
in Typical Application)
Low Switching Harmonic Content
Independent Control of Switch Voltage and
Current Slew Rates
Two 1A Current Limited Power Switches
Regulates Positive and Negative Voltages
20kHz to 250kHz Oscillator Frequency
Easily Synchronized to External Clock
Wide Input Voltage Range: 2.7V to 23V
Low Shutdown Current: 12µA Typical
Easier Layout than with Conventional Switchers
Outputs Can Be Forced to 50% Duty Cycle for
Unregulated Applications
APPLICATIONS
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Precision Instrumentation Systems
Isolated Supplies for Industrial Automation
Medical Instruments
Wireless Communications
Single Board Data Acquisition Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
Low Noise 5V to 12V Forward Push-Pull DC/DC Converter
5V
+
33µF
11
3
820pF
4
5
16.9k
6
SHDN
DUTY
SYNC
C
T
R
T
14
V
IN
COL A
COL B
PGND
LT1533
R
VSL
R
CSL
15k
10
V
C
GND
9
NFB
8
FB
2
T1
1N4148
L1
300µH
B
L2
33µH
C1
47µF
16V
A
12V
150mA
C2
33µF
20V
12V Output Noise (BW = 100MHz)
1N4148
15
Note 1
16
13
12
7
15k
15k
21.5k, 1%
+
+
A
100µV/DIV
B
2mV/DIV
1533 TA01
0.015µF
1000pF
2.49k
1%
C1: SANYO OS-CON
C2: AVX TPS TANTALUM
L1: COILTRONICS CTX300-2
L2: COILCRAFT DT1608C-333
T1: COILTRONICS CTX02-13834
NOTE 1: 25nH TRACE INDUCTANCE
OR COILCRAFT B07T
U
U
U
<100µV
P-P
2µs/DIV
1533 TA02
1
LT1533
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
NC 1
COL A 2
DUTY 3
SYNC 4
C
T
5
R
T
6
FB 7
NFB 8
16 PGND
15 COL B
14 V
IN
13 R
VSL
12 R
CSL
11 SHDN
10 V
C
9
GND
Input Voltage (V
IN
) .................................................. 30V
Switch Voltage (COL A, COL B) ............................... 30V
SHDN Pin Voltage .................................................... 30V
Feedback Pin Current ............................................ 10mA
Negative Feedback Pin Current ............................
±10mA
Storage Temperature Range ................. – 65°C to 150°C
Maximum Junction Temperature ......................... 125°C
Operating Junction Temperature Range
LT1533C ............................................... 0°C to 100°C
LT1533I ............................................ – 40°C to 100°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1533CS
LT1533IS
S PACKAGE
16-LEAD NARROW PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 100°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.9V, V
FB
= V
REF
. COL A, COL B, SHDN, NFB, DUTY pins open, unless otherwise noted.
SYMBOL
V
IN
V
IN(MIN)
I
VIN
I
VIN(OFF)
V
SHDN
I
SHDN
V
REF
I
FB
FB
REG
V
NFR
I
NFR
NFB
REG
PARAMETER
Recommended Operating Range
Minimum Input Voltage
Supply Current
Shutdown Supply Current
Shutdown Threshold
Shutdown Input Current
Reference Voltage
Feedback Input Current
Reference Voltage Line Regulation
Negative Feedback Reference Voltage
Negative Feedback Input Current
Negative Feedback Reference Voltage
Line Regulation
Measured at Feedback Pin
q
CONDITIONS
q
q
MIN
2.7
TYP
MAX
23
UNITS
V
V
mA
µA
V
µA
Supply and Protection
2.55
12
12
0.4
0.8
–2
1.235
1.215
1.250
1.250
250
0.003
– 2.550
– 37
– 2.500
– 25
0.002
0.05
1.265
1.275
900
0.03
– 2.420
2.7
18
30
1.2
2.7V
≤
V
IN
≤
23V, R
VSL
, R
CSL
, R
T
= 17k
2.7V
≤
V
IN
≤
23V, V
SHDN
= 0V
2.7V
≤
V
IN
≤
23V
q
q
q
Error Amplifiers
V
V
nA
%/V
V
µA
%/V
V
FB
= V
REF
2.7V
≤
V
IN
≤
23V
Measured at Negative Feedback Pin with
Feedback Pin Open
V
NFB
= V
NFR
2.7V
≤
V
IN
≤
23V
q
q
q
q
q
2
U
W
U
U
W W
W
LT1533
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.9V, V
FB
= V
REF
. COL A, COL B, SHDN, NFB, DUTY pins open, unless otherwise noted.
SYMBOL
g
m
I
ESK
I
ESRC
V
CLH
V
CLL
A
V
f
MAX
f
SYNC
R
SYNC
V
FBfs
DC
MAX
t
IBL
BV
R
ON
I
LIM(MAX)
PARAMETER
Error Amplifier Transconductance
Error Amplifier Sink Current
Error Amplifier Source Current
Error Amplifier Clamp Voltage
Error Amplifier Clamp Voltage
Error Amplifier Voltage Gain
Maximum Switch Frequency
Synchronization Frequency Range
SYNC Pin Input Resistance
FB Pin Threshold for Frequency Shift
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
Output Switch Breakdown Voltage
Output Switch-On Resistance
Maximum Current Limit
Short-Circuit Current Limit
2.7V
≤
V
IN
≤
23V
I
COL A
or I
COL B
= 0.75A
Duty Cycle = 15%
Duty Cycle = 40%
q
q
CONDITIONS
∆I
C
=
±25µA
q
MIN
1100
700
120
120
TYP
1500
200
200
1.33
0.1
MAX
1900
2300
350
350
UNITS
µmho
µmho
µA
µA
V
V
V/V
kHz
V
FB
= V
REF
+ 150mV, V
C
= 0.9V, V
SHDN
= 1V
V
FB
= V
REF
– 150mV, V
C
= 0.9V, V
SHDN
= 1V
High Clamp, V
FB
= 1V
Low Clamp, V
FB
= 1.5V
q
q
180
250
250
Oscillator and Sync
f
OSC
= 250kHz
5% Reduction from Nominal
DUTY Pin Open, R
VSL
= R
CSL
= 4.9k, f
OSC
= 25kHz
DUTY Pin Grounded, Forced 50% Duty Cycle
q
q
375
40
0.4
44
45.5
50.0
200
25
1
0.8
30
0.5
1.25
16
0.35
0.85
1.8
kHz
kΩ
V
%
%
ns
V
Ω
A
A
mA/A
Output Switches
∆I
IN
/∆I
SW
Supply Current Increase During
Switch-On Time
V
DUTYTH
V
SLEWR
V
SLEWF
I
SLEWR
I
SLEWF
DUTY Pin Threshold
Output Voltage Slew Rising Edge
Output Voltage Slew Falling Edge
Output Current Slew Rising Edge
Output Current Slew Falling Edge
Either A or B, R
VSL
, R
CSL
= 17k
Either A or B, R
VSL
, R
CSL
= 17k
Either A or B, R
VSL
, R
CSL
= 17k
Either A or B, R
VSL
, R
CSL
= 17k
Slew Control
11
14.5
1.3
1.3
V/µs
V/µs
A/µs
A/µs
The
q
denotes specifications that apply over the full operating
temperature range.
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1533 is designed to operate over the junction temperature
range of – 4 0°C to 125°C, but is neither tested nor guaranteed beyond 0°C
to 100°C for C grade or – 40°C to 100°C for I grade.
3
LT1533
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage vs
Temperature
2.70
0
–50
2.65
INPUT VOLTAGE (V)
SWITCH VOLTAGE (V)
∆I
LIM
(mA)
2.60
2.55
2.50
2.45
–50 –25
25 50 75 100 125 150
0
JUNCTION TEMPERATURE (°C)
1533 G01
Feedback Voltage and Input
Current vs Temperature
1.30
1.29
1.28
FEEDBACK VOLTAGE (V)
NEGATIVE FEEDBACK VOLTAGE (V)
–2.40
–2.45
–2.50
–2.55
–2.60
–2.65
–2.70
–50 –25
0
I
NFB
V
NFB
30
TRANSCONDUCTANCE (mho)
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25
0
I
FB
V
FB
0
25 50 75 100 125 150
TEMPERATURE (°C)
1533 G04
Switching Frequency vs
Feedback Pin Voltage
120
SWITCHING FREQUENCY (% TYPICAL)
ERROR AMPLIFIER OUTPUT (µA)
100
80
60
40
20
0
0
0.1
0.3
0.4
0.5
0.2
FEEDBACK PIN VOLTAGE (V)
0.6
1533 G07
200
100
0
–100
–200
–300
–400
25°C
V
C
PIN VOLTAGE (V)
4
U W
Change in I
LIM
vs DC
0.7
0.6
25°C
125°C
Switch Voltage Drop
125°C
–100
–150
–200
–250
–300
0.5
0.4
0.3
0.2
0.1
0
85°C
25°C
0
10
20
30
DUTY CYCLE (%)
40
50
1533 G02
0
0.2
0.8
0.6
SWITCH CURRENT (A)
0.4
1.0
1533 G03
Negative Feedback Voltage and
Input Current vs Temperature
2.0
1.8
FEEDBACK INPUT CURRENT (µA)
Error Amplifier Transconductance
vs Temperature
35
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
–50 –25
0
25
50
75
100 125 150
1533 G06
–2.30
–2.35
g
m
=
∆I
VC
/∆V
FB
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
NFB INPUT CURRENT (µA)
25
20
15
25 50 75 100 125 150
TEMPERATURE (°C)
1533 G05
TEMPERATURE (°C)
Error Amplifier Output Current
500
400
300
–40°C
125°C
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
V
C
Pin Threshold and Clamp
Voltage vs Temperature
V
C
PIN CLAMP
VOLTAGE
V
C
PIN
THRESHOLD
–500
–400 –300 –200 –100 0 100 200 300 400
FEEDBACK PIN VOLTAGE FROM NOMINAL (mV)
1533 G08
0
–50 –25
0
25
50
75
100 125
1533 G09
TEMPERATURE (°C)
LT1533
PIN FUNCTIONS
COL A, COL B (Pins 2, 15):
These are the output collectors
of the power switches. Their emitters return to PGND
through a common sense resistor. COL A and
COL B are alternately turned on out of phase. Large
currents flow into these pins so it is desirable to keep
external trace lengths short to minimize radiation. The
collectors can be tied together for simple boost applica-
tions.
DUTY (Pin 3):
Tying the DUTY pin to ground will force the
outputs to switch with a 50% duty cycle. The DUTY pin
must float if not used.
SYNC (Pin 4):
The SYNC pin can be used to synchronize
the oscillator to an external clock (see Oscillator Sync in
Applications Information section for more details). The
SYNC pin may either be floated or tied to ground if not
used.
C
T
(Pin 5):
The oscillator capacitor pin is used in conjunc-
tion with R
T
to set the oscillator frequency. For R
T
= 16.9k,
C
T(NF)
= 129/f
OSC(kHz)
R
T
(Pin 6):
The oscillator resistor pin is used to set the
charge and discharge currents of the oscillator capacitor.
The nominal value is 16.9k. It is possible to adjust this
resistance
±25%
to get a more accurate oscillator fre-
quency.
FB (Pin 7):
The feedback pin is used for positive voltage
sensing and oscillator frequency shifting during start-up
and short-circuit conditions. It is the inverting input to the
error amplifier. The noninverting input of this amplifier
connects internally to a 1.25V reference. This pin should
be left open if not used.
NFB (Pin 8):
The negative voltage feedback pin is used for
sensing a negative output voltage. The pin is connected to
the inverting input of the negative feedback amplifier
through a 100k source resistor. The negative feedback
amplifier provides a gain of – 0.5 to the feedback amplifier.
The nominal regulation point would be – 2.5V on NFB. This
pin should be left open if not used.
GND (Pin 9):
Signal Ground. The internal error amplifier,
negative feedback amplifier, oscillator, slew control cir-
cuitry and the bandgap reference are referred to this
ground. Keep the connection to the feedback divider and
V
C
compensation network free of large ground currents.
V
C
(Pin 10):
The compensation pin is used for frequency
compensation and current limiting. It is the output of the
error amplifier and the input of the current comparator.
Loop frequency compensation can be performed with an
RC network connected from the V
C
pin to ground.
SHDN (Pin 11):
The shutdown pin is used for disabling the
switcher. Grounding this pin will disable all internal cir-
cuitry. Normally this output can be tied high (to V
IN
) or may
be left floating.
R
CSL
(Pin 12):
A resistor to ground sets the current slew
rate for the collectors A and B. The minimum resistor value
is 3.9k and the maximum value is 68k. Current slew will be
approximately:
I
SLEW(A/µs)
= 33/R
CSL(kΩ)
R
VSL
(Pin 13):
A resistor to ground sets the voltage slew
rate for the collectors A and B. The minimum resistor value
is 3.9k and the maximum value is 68k. Voltage slew will be
approximately:
V
SLEW(V/µs)
= 220/R
VSL(kΩ)
V
IN
(Pin 14):
Input Supply Pin. Bypass this pin with a
≥
4.7µF low ESR capacitor. When V
IN
is below 2.55V the
part will go into undervoltage lockout where it will stop
output switching and pull the V
C
pin low.
PGND (Pin 16):
Power Switch Ground. This ground comes
from the emitters of the power switches. In normal opera-
tion this pin should have approximately 25nH inductance
to ground. This can be done by trace inductance (approxi-
mately 1") or with wire or a specific inductive component.
This inductance ensures stability in the current slew
control loop during turn-off. Too much inductance (>50nH)
may produce oscillation on the output voltage slew edges.
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Analogue and Mixed Signal
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