1
LT1506
4.5A, 500kHz Step-Down
Switching Regulator
s
Constant 500kHz Switching Frequency
s
Easily Synchronizable
s
Operates with Input as Low as 4V
s
Uses All Surface Mount Components
s
Inductor Size Reduced to 1.8
H
s
Saturating Switch Design: 0.07
s
Shutdown Current: 20
A
s
Cycle-by-Cycle Current Limiting
FEATURES
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 20
A. Synchronization allows an external logic level sig-
nal to increase the internal oscillator from 580kHz to 1MHz.
DESCRIPTIO
N
U
s
Portable Computers
s
Battery-Powered Systems
s
Battery Charger
s
Distributed Power
APPLICATIO
N
S
U
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
N
U
LOAD CURRENT (A)
0
EFFICIENCY (%)
90
85
80
75
70
2.0
2.5
3.0
3.5
1506 TA02
0.5
1.0
1.5
4.0
V
OUT
= 3.3V
V
IN
= 5V
L = 10
H
Efficiency vs Load Current
5V to 3.3V Down Converter
BOOST
LT1506-3.3
V
IN
OUTPUT
3.3V
4A
INPUT
5V
1506 TA01
C2
0.68
F
C
C
1.5nF
D1
MBRS330T3
C1
100
F, 10V
SOLID
TANTALUM
C3
10
F TO
50
F
CERAMIC
D2
1N914
L1
5
H
V
SW
SENSE
SHDN
OPEN
OR
HIGH
= ON
GND
V
C
+
+
2
LT1506
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
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
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
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Feedback Voltage (Adjustable)
2.39
2.42
2.45
V
All Conditions
q
2.36
2.48
V
Sense Voltage (Fixed 3.3V)
3.25
3.3
3.35
V
All Conditions
q
3.23
3.37
V
SENSE Pin Resistance
4
6.6
9.5
k
Reference Voltage Line Regulation
4.3V
V
IN
15V
0.01
0.03
%/ V
Feedback Input Bias Current
q
0.5
2
A
Error Amplifier Voltage Gain
(Notes 2, 8)
200
400
Error Amplifier Transconductance
I (V
C
) =
10
A (Note 8)
1500
2000
2700
Mho
q
1000
3100
Mho
V
C
Pin to Switch Current Transconductance
5.3
A/ V
Error Amplifier Source Current
V
FB
= 2.1V or V
SENSE
= 2.9V
q
140
225
320
A
Error Amplifier Sink Current
V
FB
= 2.7V or V
SENSE
= 3.7V
q
140
225
320
A
V
C
Pin Switching Threshold
Duty Cycle = 0
0.9
V
V
C
Pin High Clamp
2.1
V
Switch Current Limit
V
C
Open, V
FB
= 2.1V or V
SENSE
= 2.9V, DC
50%
q
4.5
6
8.5
A
Slope Compensation
DC = 80%
0.8
A
T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER PART
NUMBER
ORDER PART
NUMBER
LT1506CR
LT1506CR-3.3
LT1506CR-SYNC
LT1506CR-3.3 SYNC
LT1506IR
LT1506IR-3.3
LT1506IR-SYNC
LT1506IR-3.3 SYNC
T
JMAX
= 125
C,
JA
= 30
C/W
WITH PACKAGE SOLDERED TO 0.5 SQUARE INCH
COPPER AREA OVER BACKSIDE GROUND PLANE OR
INTERNAL POWER PLANE.
JA
CAN VARY FROM 20
C/W
TO > 40
C/W DEPENDING ON MOUNTING TECHNIQUES
1506I
506I33
1506
150633
S8 PART MARKING
LT1506CS8
LT1506CS8-3.3
LT1506IS8
LT1506IS8-3.3
*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.
FB OR SENSE*
BOOST
V
IN
GND
V
SW
SYNC OR SHDN*
V
C
R PACKAGE
7-LEAD PLASTIC DD PAK
FRONT VIEW
TAB
IS
GND
7
6
5
4
3
2
1
1
2
3
4
8
7
6
5
TOP VIEW
S8 PACKAGE
8-LEAD PLASTIC SO
V
IN
BOOST
GND**
V
SW
SYNC
SHDN
V
C
FB OR
SENSE*
JA
= 80
C/ W
**WITH FUSED (GND) GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
(Note 1)
3
LT1506
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch On Resistance (Note 7)
I
SW
= 4.5A
0.07
0.1
q
0.13
Maximum Switch Duty Cycle
V
FB
= 2.1V or V
SENSE
= 2.9V
90
93
%
q
86
93
%
Switch Frequency
V
C
Set to Give 50% Duty Cycle
460
500
540
kHz
q
440
560
kHz
Switch Frequency Line Regulation
4.3V
V
IN
15V
q
0
0.15
%/ V
Frequency Shifting Threshold on FB Pin
f = 10kHz
q
0.8
1.0
1.3
V
Minimum Input Voltage (Note 3)
q
4.0
4.3
V
Minimum Boost Voltage (Note 4)
I
SW
4.5A
q
2.3
3.0
V
Boost Current (Note 5)
I
SW
= 1A
q
20
35
mA
I
SW
= 4.5A
q
90
140
mA
Input Supply Current (Note 6)
q
3.8
5.4
mA
Shutdown Supply Current
V
SHDN
= 0V, V
SW
= 0V, V
C
Open
15
50
A
q
75
A
Lockout Threshold
V
C
Open
q
2.3
2.38
2.46
V
Shutdown Thresholds
V
C
Open Device Shutting Down
q
0.13
0.37
0.60
V
Device Starting Up
q
0.25
0.45
0.7
V
Synchronization Threshold
q
1.5
2.2
V
Synchronizing Range
580
1000
kHz
SYNC Pin Input Resistance
40
k
T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
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.
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.
4
LT1506
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Feedback Pin Voltage
TEMPERATURE (
C)
50
2.430
2.425
2.420
2.415
2.410
100
1506 G03
25
0
25
50
75
125
FEEDBACK VOLTAGE (V)
Switch Peak Current Limit
DUTY CYCLE (%)
0
SWITCH PEAK CURRENT (A)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
80
1506 G02
20
40
60
100
TYPICAL
MINIMUM
Lockout and Shutdown
Thresholds
Shutdown Pin Bias Current
Shutdown Supply Current
JUNCTION TEMPERATURE (
C)
50
2.40
2.36
2.32
0.8
0.4
0
25
75
1506 G05
25
0
50
100
125
SHUTDOWN PIN VOLTAGE (V)
LOCKOUT
START-UP
SHUTDOWN
INPUT VOLTAGE (V)
0
INPUT SUPPLY CURRENT (
A)
25
20
15
10
5
0
5
10
15
1506 G06
V
SHDN
= 0V
Shutdown Supply Current
SHUTDOWN VOLTAGE (V)
0
INPUT SUPPLY CURRENT (
A)
70
60
50
40
30
20
10
0
0.1
0.2
0.3
0.4
1506 G07
V
IN
= 10V
FREQUENCY (Hz)
GAIN (
Mho)
PHASE (DEG)
3000
2500
2000
1500
1000
500
200
150
100
50
0
50
100
10k
100k
10M
1506 G09
1k
1M
GAIN
PHASE
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
OUT
200k
C
OUT
12pF
V
C
R
LOAD
= 50
V
FB 2
10
3
)
(
Error Amplifier Transconductance
Error Amplifier Transconductance
JUNCTION TEMPERATURE (
C)
50
TRANSCONDUCTANCE (
Mho)
2500
2000
1500
1000
500
0
0
50
75
1506 G08
25
25
100
125
Minimum Input Voltage
with 3.3V Output
LOAD CURRENT (mA)
1
4.1
INPUT VOLTAGE (V)
4.3
4.5
4.7
10
100
1000
1506 G12
3.9
3.7
3.5
3.3
TEMPERATURE (
C)
50
500
400
300
200
8
4
0
25
75
1506 G04
25
0
50
100
125
CURRENT (
A)
AT 0.37V SHUTDOWN THRESHOLD.
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
A
AT 2.38V LOCKOUT THRESHOLD
5
LT1506
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Frequency Foldback
FEEDBACK PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz) OR CURRENT (
A) 500
400
300
200
100
0
2.0
1506 G10
0.5
1.0
1.5
2.5
SWITCHING
FREQUENCY
FEEDBACK PIN
CURRENT
Inductor Core Loss for 3.3V Output
INDUCTANCE (
H)
0
2
CORE LOSS (W)
1.0
0.1
0.01
0.001
4
6
8
10
1506 G01
TYPE 52
Metglas
Kool M
PERMALLOY
= 125
Switching Frequency
TEMPERATURE (
C)
50
550
540
530
520
510
500
490
480
470
460
450
100
1506 G11
25
0
25
50
75
125
FREQUENCY (kHz)
BOOST Pin Current
SWITCH CURRENT (A)
0
0
BOOST PIN CURRENT (mA)
10
20
40
30
60
50
1
2
3
4
5
1506 G14
80
70
100
90
DUTY CYCLE = 100%
Maximum Load Current
at V
OUT
= 3.3V
INPUT VOLTAGE (V)
4
4.0
4.2
4.4
12
1506 G13
3.8
3.6
6
10
8
14
3.4
3.2
3.0
LOAD CURRENT (A)
L= 10
H
L= 5
H
L= 3
H
L= 1.8
H
Maximum Load Current
at V
OUT
= 5V
INPUT VOLTAGE (V)
5
2.6
LOAD CURRENT (A)
2.8
3.2
3.4
3.6
9
13
15
4.4
1506 G17
3.0
7
11
3.8
4.0
4.2
L= 10
H
L= 5
H
L= 3
H
L= 1.8
H
Switch Voltage Drop
SWITCH CURRENT (A)
0
SWITCH VOLTAGE (mV)
250
200
400
125
C
25
C
350
300
4
5
1506 G16
150
100
50
0
1
2
3
500
450
40
C
V
C
Pin Shutdown Threshold
JUNCTION TEMPERATURE (
C)
50
1.4
1.2
1.0
0.8
0.6
0.4
100
1506 G15
25
0
25
50
75
125
THRESHOLD VOLTAGE (V)
SHUTDOWN
OUTPUT VOLTAGE (%)
0
5
6
7
80
1506 G18
4
3
20
40
60
100
2
1
0
OUTPUT CURRENT (A)
FOLDBACK
CHARACTERISTICS
CURRENT
SOURCE
LOAD
RESISTOR
LOAD
MOS LOAD
POSSIBLE UNDESIRED
STABLE POINT FOR
CURRENT SOURCE
LOAD*
Current Limit Foldback
Kool M
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.
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