LT1506
4.5A, 500kHz Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1506 is a 500kHz monolithic buck mode switching
regulator functionally identical to the LT1374 but optimized
for lower input voltage applications. It will operate over a
4V to 15V input range compared with 5.5V to 25V for the
LT1374. A 4.5A switch is included on the die along with all
the necessary oscillator, control and logic circuitry. High
switching frequency allows a considerable reduction in the
size of external components. The topology is current mode
for fast transient response and good loop stability. Both
fixed output voltage and adjustable parts are available.
A special high speed bipolar process and new design tech-
niques achieve high efficiency at high switching frequency.
Efficiency is maintained over a wide output current range
by keeping quiescent supply current to 4mA and by utiliz-
ing a supply boost capacitor to saturate the power switch.
The LT1506 fits into standard 7-pin DD and fused lead
SO-8 packages. Full cycle-by-cycle short-circuit protection
and thermal shutdown are provided. Standard surface
mount external parts are used, including the inductor and
capacitors. There is the optional function of shutdown or
synchronization. A shutdown signal reduces supply current
to 20µA. Synchronization allows an external logic level sig-
nal to increase the internal oscillator from 580kHz to 1MHz.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Constant 500kHz Switching Frequency
Easily Synchronizable
Operates with Input as Low as 4V
Uses All Surface Mount Components
Inductor Size Reduced to 1.8
µ
H
Saturating Switch Design: 0.07Ω
Shutdown Current: 20µA
Cycle-by-Cycle Current Limiting
APPLICATIONS
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Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
TYPICAL APPLICATION
5V to 3.3V Down Converter
D2
1N914
C2
0.68µF
INPUT
5V
BOOST
V
IN
V
SW
LT1506-3.3
SHDN
GND
SENSE
V
C
C
C
1.5nF
D1
MBRS330T3
L1
5µH
OUTPUT
3.3V
4A
EFFICIENCY (%)
90
V
OUT
= 3.3V
V
IN
= 5V
L = 10µH
85
80
C3
10µF TO
50µF
CERAMIC
+
OPEN
OR
HIGH
= ON
75
+
C1
100µF, 10V
SOLID
TANTALUM
1506 TA01
70
0
0.5
1.0
1.5 2.0 2.5 3.0
LOAD CURRENT (A)
3.5
4.0
U
U
U
Efficiency vs Load Current
1506 TA02
1
LT1506
ABSOLUTE
MAXIMUM
RATINGS
Input Voltage .......................................................... 16V
BOOST Pin Above Input Voltage ............................. 15V
SHDN Pin Voltage ..................................................... 7V
FB Pin Voltage (Adjustable Part) ............................ 3.5V
FB Pin Current (Adjustable Part) ............................ 1mA
Sense Voltage (Fixed 3.3V Part) ............................... 5V
PACKAGE/ORDER INFORMATION
FRONT VIEW
7
6
5
4
3
2
1
FB OR SENSE*
BOOST
V
IN
GND
V
SW
SYNC OR SHDN*
V
C
ORDER PART
NUMBER
LT1506CR
LT1506CR-3.3
LT1506CR-SYNC
LT1506CR-3.3 SYNC
LT1506IR
LT1506IR-3.3
LT1506IR-SYNC
LT1506IR-3.3 SYNC
V
IN
1
BOOST 2
FB OR
3
SENSE*
GND** 4
TAB
IS
GND
R PACKAGE
7-LEAD PLASTIC DD PAK
T
JMAX
= 125°C,
θ
JA
= 30°C/W
WITH PACKAGE SOLDERED TO 0.5 SQUARE INCH
COPPER AREA OVER BACKSIDE GROUND PLANE OR
INTERNAL POWER PLANE.
θ
JA
CAN VARY FROM 20°C/W
TO > 40°C/W DEPENDING ON MOUNTING TECHNIQUES
*Default is the adjustable output voltage device with FB pin and shutdown function. Option -3.3 replaces FB with SENSE pin for fixed 3.3V output
applications. -SYNC replaces SHDN with SYNC pin for applications requiring synchronization. Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
T
J
= 25°C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Feedback Voltage (Adjustable)
Sense Voltage (Fixed 3.3V)
All Conditions
SENSE Pin Resistance
Reference Voltage Line Regulation
Feedback Input Bias Current
Error Amplifier Voltage Gain
Error Amplifier Transconductance
V
C
Pin to Switch Current Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
V
C
Pin Switching Threshold
V
C
Pin High Clamp
Switch Current Limit
Slope Compensation
4.3V
≤
V
IN
≤
15V
q
q
CONDITIONS
All Conditions
q
(Notes 2, 8)
∆I
(V
C
) =
±10µA
(Note 8)
q
V
FB
= 2.1V or V
SENSE
= 2.9V
V
FB
= 2.7V or V
SENSE
= 3.7V
Duty Cycle = 0
V
C
Open, V
FB
= 2.1V or V
SENSE
= 2.9V, DC
≤
50%
DC = 80%
2
U
U
W
W W
U
W
(Note 1)
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1506C ............................................... 0°C to 125° C
LT1506I ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
8 V
SW
7 SYNC
6 SHDN
5 V
C
ORDER PART
NUMBER
LT1506CS8
LT1506CS8-3.3
LT1506IS8
LT1506IS8-3.3
S8 PART MARKING
1506
150633
1506I
506I33
S8 PACKAGE
8-LEAD PLASTIC SO
θ
JA
= 80°C/ W
**WITH FUSED (GND) GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
MIN
2.39
2.36
3.25
3.23
4
TYP
2.42
3.3
6.6
0.01
0.5
400
2000
5.3
225
225
0.9
2.1
6
0.8
MAX
2.45
2.48
3.35
3.37
9.5
0.03
2
2700
3100
320
320
UNITS
V
V
V
V
kΩ
%/ V
µA
µMho
µMho
A/ V
µA
µA
V
V
A
A
200
1500
1000
140
140
q
q
q
4.5
8.5
LT1506
ELECTRICAL CHARACTERISTICS
T
J
= 25°C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Switch On Resistance (Note 7)
Maximum Switch Duty Cycle
Switch Frequency
Switch Frequency Line Regulation
Frequency Shifting Threshold on FB Pin
Minimum Input Voltage (Note 3)
Minimum Boost Voltage (Note 4)
Boost Current (Note 5)
Input Supply Current (Note 6)
Shutdown Supply Current
Lockout Threshold
Shutdown Thresholds
Synchronization Threshold
Synchronizing Range
SYNC Pin Input Resistance
The
q
denotes specifications which 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:
Gain is measured with a V
C
swing equal to 200mV above the
switching threshold level to 200mV below the upper clamp level.
Note 3:
Minimum input voltage is not measured directly, but is guaranteed
by other tests. It is defined as the voltage where internal bias lines are still
regulated so that the reference voltage and oscillator frequency remain
constant. Actual minimum input voltage to maintain a regulated output will
depend on output voltage and load current. See Applications Information.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
CONDITIONS
I
SW
= 4.5A
q
MIN
TYP
0.07
93
93
500
0
1.0
4.0
2.3
20
90
3.8
15
2.38
0.37
0.45
1.5
40
MAX
0.1
0.13
V
FB
= 2.1V or V
SENSE
= 2.9V
q
V
C
Set to Give 50% Duty Cycle
q
90
86
460
440
0.8
4.3V
≤
V
IN
≤
15V
∆f
= 10kHz
I
SW
≤
4.5A
I
SW
= 1A
I
SW
= 4.5A
V
SHDN
= 0V, V
SW
= 0V, V
C
Open
q
q
q
q
q
q
q
q
V
C
Open
V
C
Open Device Shutting Down
Device Starting Up
q
q
q
q
2.3
0.13
0.25
580
540
560
0.15
1.3
4.3
3.0
35
140
5.4
50
75
2.46
0.60
0.7
2.2
1000
UNITS
Ω
Ω
%
%
kHz
kHz
%/ V
V
V
V
mA
mA
mA
µA
µA
V
V
V
V
kHz
kΩ
Note 5:
Boost current is the current flowing into the boost pin with the pin
held 5V above input voltage. It flows only during switch on time.
Note 6:
Input supply current is the bias current drawn by the input pin
with switching disabled.
Note 7:
Switch on resistance is calculated by dividing V
IN
to V
SW
voltage
by the forced current (4.5A). See Typical Performance Characteristics for
the graph of switch voltage at other currents.
Note 8:
Transconductance and voltage gain refer to the internal amplifier
exclusive of the voltage divider. To calculate gain and transconductance
refer to SENSE pin on fixed voltage parts. Divide values shown by the ratio
V
OUT
/2.42.
3
LT1506
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
with 3.3V Output
4.7
4.5
SWITCH PEAK CURRENT (A)
INPUT VOLTAGE (V)
4.3
4.1
3.9
3.7
3.5
3.3
1
10
100
LOAD CURRENT (mA)
1000
1506 G12
5.5
5.0
MINIMUM
4.5
4.0
3.5
3.0
0
20
60
40
DUTY CYCLE (%)
80
100
1506 G02
FEEDBACK VOLTAGE (V)
Shutdown Pin Bias Current
–500
– 400
– 300
– 200
–8
AT 2.38V LOCKOUT THRESHOLD
–4
0
–50 –25
AT 0.37V SHUTDOWN THRESHOLD.
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
µA
2.40
2.36
INPUT SUPPLY CURRENT (µA)
SHUTDOWN PIN VOLTAGE (V)
CURRENT (µA)
50
25
75
0
TEMPERATURE (°C)
Shutdown Supply Current
70
60
50
40
30
20
10
2500
V
IN
= 10V
TRANSCONDUCTANCE (µMho)
INPUT SUPPLY CURRENT (µA)
GAIN (µMho)
0
0
0.1
0.2
0.3
SHUTDOWN VOLTAGE (V)
0.4
1506 G07
4
U W
100
1506 G04
Switch Peak Current Limit
6.5
6.0
TYPICAL
2.425
2.430
Feedback Pin Voltage
2.420
2.415
2.410
– 50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
1506 G03
Lockout and Shutdown
Thresholds
25
LOCKOUT
20
Shutdown Supply Current
V
SHDN
= 0V
2.32
15
0.8
START-UP
0.4
SHUTDOWN
10
5
125
0
50
100
25
75
–50 –25
0
JUNCTION TEMPERATURE (°C)
0
125
0
5
10
INPUT VOLTAGE (V)
15
1506 G06
1506 G05
Error Amplifier Transconductance
3000
Error Amplifier Transconductance
200
PHASE
2500
GAIN
100
V
C
C
OUT
12pF
2000
150
PHASE (DEG)
1500
2000
1000
1500
V
FB
2
×
10
–3
(
)
R
OUT
200k
50
500
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
R
LOAD
= 50Ω
0
50
0
75 100
25
–50 –25
JUNCTION TEMPERATURE (°C)
125
500
100
1k
10k
100k
FREQUENCY (Hz)
1M
–50
10M
1506 G09
1506 G08
LT1506
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback
SWITCHING FREQUENCY (kHz) OR CURRENT (µA)
500
550
540
400
FREQUENCY (kHz)
300
510
500
490
480
470
CORE LOSS (W)
SWITCHING
FREQUENCY
200
100
FEEDBACK PIN
CURRENT
0
0
0.5
1.5
2.0
1.0
FEEDBACK PIN VOLTAGE (V)
2.5
1506 G10
Maximum Load Current
at V
OUT
= 5V
4.4
L= 10µH
4.2
4.0
LOAD CURRENT (A)
3.8
3.6
3.4
3.2
3.0
2.8
2.6
5
7
11
9
INPUT VOLTAGE (V)
13
15
1506 G17
LOAD CURRENT (A)
4.0
3.8
3.6
3.4
L= 1.8µH
3.2
3.0
4
6
8
10
12
14
1506 G13
BOOST PIN CURRENT (mA)
L= 5µH
L= 3µH
L= 1.8µH
Current Limit Foldback
7
6
FOLDBACK
CHARACTERISTICS
THRESHOLD VOLTAGE (V)
5
4
3
2
SWITCH VOLTAGE (mV)
OUTPUT CURRENT (A)
POSSIBLE UNDESIRED
CURRENT
STABLE POINT FOR
SOURCE
CURRENT SOURCE
LOAD
LOAD*
RESISTOR
LOAD
MOS LOAD
1
0
0
20
40
60
80
100
1506 G18
OUTPUT VOLTAGE (%)
Kool Mµ is a registered trademark of Magnetics, Inc.
Metglas is a registered trademark of AlliedSignal Inc.
*See “More Than Just Voltage Feedback” in the Applications Information section.
U W
Switching Frequency
1.0
Inductor Core Loss for 3.3V Output
530
520
0.1
TYPE 52
Kool Mµ
®
0.01
PERMALLOY
µ
= 125
Metglas
®
460
450
– 50
–25
0
25
50
75
100
125
0.001
0
2
4
6
INDUCTANCE (µH)
8
10
TEMPERATURE (°C)
1506 G11
1506 G01
Maximum Load Current
at V
OUT
= 3.3V
4.4
L= 10µH
4.2
L= 5µH
100
90
80
70
60
50
40
30
20
10
0
BOOST Pin Current
DUTY CYCLE = 100%
L= 3µH
0
1
INPUT VOLTAGE (V)
3
2
4
SWITCH CURRENT (A)
5
1506 G14
V
C
Pin Shutdown Threshold
1.4
SHUTDOWN
1.2
Switch Voltage Drop
500
450
400
350
300
250
200
150
100
50
25°C
125°C
1.0
– 40°C
0.8
0.6
0.4
–50
0
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
125
0
1
2
4
3
SWITCH CURRENT (A)
5
1506 G16
1506 G15
5
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