LT1507
500kHz Monolithic
Buck Mode Switching Regulator
FEATURES
s
s
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Constant 500kHz Switching Frequency
Uses All Surface Mount Components
Operates with Inputs as Low as 4V
Saturated Switch Design (0.3Ω)
Cycle-by-Cycle Current Limiting
Easily Synchronizable
Inductor Size as Low as 2µH
Shutdown Current: 20µA
with all the necessary oscillator, control and logic cir-
cuitry. 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 (3.3V) and adjustable
parts are available.
A special high speed bipolar process and new design
techniques allow this regulator to achieve high efficiency
at a high switching frequency. Efficiency is maintained
over a wide output current range by keeping quiescent
supply current to 4mA and by utilizing a supply boost
capacitor to allow the NPN power switch to saturate. A
shutdown signal will reduce supply current to 20µA. The
LT1507 can be externally synchronized from 570kHz to
1MHz with logic level inputs.
The LT1507 fits into standard 8-pin SO and PDIP pack-
ages. Temperature rise is kept to a minimum by the high
efficiency design. Full cycle-by-cycle short-circuit protec-
tion and thermal shutdown are provided. Standard surface
mount external parts are used including the inductor and
capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIONS
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Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
DESCRIPTION
The LT
®
1507 is a 500kHz monolithic buck mode switching
regulator, functionally identical to the LT1375 but opti-
mized for lower input voltage applications. It will operate
over a 4V to 15V input range, compared with 5.5V to 25V
for the LT1375. A 1.5A switch is included on the die along
TYPICAL APPLICATION
5V to 3.3V Volt Down Converter
D2
†
1N914
C2
0.1µF
V
SW
LT1507-3.3
DEFAULT
(OPEN)
= ON
SHDN
GND
SENSE
V
C
C
C
3.3nF
D1
1N5818
L1***
5µH
5V
C3*
47µF
16V
TANTALUM
V
IN
EFFICIENCY (%)
BOOST
+
* AVX TPSD477M016R0150 OR SPRAGUE 593D EQUIVALENT.
RIPPLE CURRENT RATING
≥
0.6A
** AVX TPSD108M010R0100 OR SPRAGUE 593D EQUIVALENT
*** COILTRONICS CTX5-1. SUBSTITUTION UNITS SHOULD BE RATED
AT
≥
1.25A, USING LOW LOSS CORE MATERIAL
†
SEE BOOST PIN CONSIDERATIONS IN APPLICATIONS INFORMATION
SECTION FOR ALTERNATIVE D2 CONNECTION
U
U
U
5V to 3.3V Efficiency
100
V
IN
= 5V
V
OUT
= 3.3V
90
OUTPUT
3.3V
1.25A
80
70
+
C1**
100µF
10V
TANTALUM
60
50
0
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
LT1507 • TA02
1
LT1507
ABSOLUTE
MAXIMUM
RATINGS
Input Voltage ........................................................... 16V
Boost Pin Voltage .................................................... 25V
Shutdown 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 Ambient Temperature Range
LT1507C .................................................. 0°C to 70°C
LT1507I .............................................. – 40°C to 85°C
Max Operating Junction Temperature................... 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
TOP VIEW
BOOST 1
V
IN
2
V
SW
3
SHDN 4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
V
C
FB/SENSE
GND
SYNC
S8 PACKAGE
8-LEAD PLASTIC SO
LT1507CN8
LT1507CN8-3.3
LT1507CS8
LT1507CS8-3.3
LT1507IN8
LT1507IN8-3.3
LT1507IS8
LT1507IS8-3.3
S8 PART MARKING
1507
15073
1507I
1507I3
T
JMAX
= 125°C,
θ
JA
= 80°C/W TO 120°C/ W (N)
T
JMAX
= 125°C,
θ
JA
= 120°C/W TO 170°C/ W (S)
DEPENDING ON PC BOARD LAYOUT
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
T
J
= 25°C, V
IN
= 5V, V
C
= 1.5V, boost open, switch open unless otherwise specified.
PARAMETER
Reference Voltage (Adjustable)
Sense Voltage (3.3V)
All Conditions
Sense Pin Resistance
Reference Voltage Line Regulation
FB Input Bias Current
Error Amplifier Voltage Gain (Note 8)
Error Amplifier Transconductance (Note 8)
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
Switch On Resistance (Note 6)
Maximum Switch Duty Cycle
4.3V
≤
V
IN
≤
15V
(Note 1)
∆I(V
C
) =
±10µA
q
q
q
q
q
CONDITIONS
All Conditions
q
MIN
2.39
2.36
3.25
3.23
4.0
TYP
2.42
3.3
6.6
0.01
0.5
400
2000
MAX
2.45
2.48
3.35
3.37
9.5
0.03
2
2700
3000
UNITS
V
V
V
V
kΩ
%/V
µA
µmho
µmho
A/V
µA
mA
V
V
A
A
Ω
Ω
%
%
150
1500
1100
V
FB
= 2.1V or V
SENSE
= 2.9V
V
FB
= 2.7V or V
SENSE
= 3.7V
Duty Cycle = 0
V
FB
= 2.1V or V
SENSE
= 2.9V
V
C
Open, V
FB
= 2.1V or V
SENSE
= 2.9V
V
IN
≥
5V, V
BOOST
= V
IN
+ 5V
I
SW
= 1.5A, V
BOOST
= V
IN
+ 5V
V
FB
= 2.1V or V
SENSE
= 2.9V
q
150
DC
≤
50%
DC = 80%
q
q
q
q
1.50
1.35
2
225
2
0.9
2.1
2
0.3
320
3
3
0.4
0.5
90
86
93
93
2
U
W
U
U
W W
W
LT1507
ELECTRICAL CHARACTERISTICS
T
J
= 25°C, V
IN
= 5V, V
C
= 1.5V, boost open, switch open unless otherwise specified.
PARAMETER
Switch Frequency
CONDITIONS
V
C
Set to Give 50% Duty Cycle
– 25°C
≤
T
J
≤
125°C
T
J
≤
–25°C
4.3V
≤
V
IN
≤
15V
∆f
= 10kHz
I
SW
≤
1.5A
V
BOOST
= V
IN
+ 5V
MIN
460
440
440
q
q
q
q
TYP
500
Switch Frequency Line Regulation
Frequency Shifting Threshold on F
B
Pin
Minimum Input Voltage (Note 2)
Minimum Boost Voltage (Note 3)
Boost Current (Note 4)
0.8
I
SW
= 500mA, –25°C
≤
T
J
≤
125°C
T
J
≤
–25°C
I
SW
= 1.5A, –25°C
≤
T
J
≤
125°C
T
J
≤
–25°C
q
0.05
1.0
4
3
12
25
3.8
15
Input Supply Current (Note 5)
Shutdown Supply Current
Lockout Threshold
Shutdown Threshold
Minimum Synchronizing Amplitude
Synchronizing Frequency Range (Note 7)
V
SHDN
= 0V, V
IN
≤
12V
V
SW
= 0V, V
C
Open
V
C
Open
V
C
Open
Device Shutting Down
Device Starting Up
q
q
q
q
q
2.3
0.15
0.25
580
2.38
0.37
0.45
1.5
MAX
540
560
570
0.15
1.3
4.3
3.5
22
25
35
40
5.4
50
75
2.46
0.70
0.70
2.2
1000
UNITS
kHz
kHz
kHz
%/V
V
V
V
mA
mA
mA
mA
mA
µA
µA
V
V
V
V
kHz
The
q
denotes specifications which apply over the operating temperature
range.
Note 1:
Gain is measured with a V
C
swing equal to 200mV above the low
clamp level to 200mV below the upper clamp level.
Note 2:
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 3:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 4:
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 5:
Input supply current is the bias current drawn by the V
IN
pin when
the SHDN pin is held at 1V (switching disabled).
Note 6:
Switch ON resistance is calculated by dividing V
IN
to V
SW
voltage
by the forced current (1.5A). See Typical Performance Characteristics for
the graph of switch voltage at other currents.
Note 7:
For synchronizing frequency above 700kHz, with duty cycles
above 50%, external slope compensation may be needed. See Applications
Information.
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.
TYPICAL PERFORMANCE CHARACTERISTICS
V
C
Pin Shutdown Threshold
1.4
THRESHOLD VOLTAGE (V)
FEEDBACK VOLTAGE (V)
1.2
EFFICIENCY (%)
1.0
0.8
0.6
0.4
–50
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
U W
LT1507 • TPC01
Switch Peak Current Limit
100
V
IN
= 5V
V
OUT
= 3.3V
90
2.43
2.44
Feedback Pin Voltage and Current
2.0
1.5
CURRENT (µA)
80
2.42
VOLTAGE
1.0
70
CURRENT
2.41
0.5
60
50
125
0
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
2.40
–50
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
0
125
LT1507 • TA02
LT1507 • TPC03
3
LT1507
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Pin Bias Current
500
400
300
200
8
4
0
–50 –25
CURRENT REQUIRED TO FORCE SHUTDOWN
(FLOWS OUT OF PIN). AFTER SHUTDOWN,
CURRENT DROPS TO A FEW
µA
2.40
STANDBY
2.36
INPUT SUPPLY CURRENT (µA)
SHUTDOWN PIN VOLTAGE (V)
CURRENT (µA)
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
50
25
75
0
TEMPERATURE (°C)
Shutdown Supply Current
150
125
100
75
50
25
0
V
IN
= 10V
2500
INPUT SUPPLY CURRENT (µA)
TRANSCONDUCTANCE (µmho)
GAIN (µmho)
0
0.1
0.2
0.3
0.4
SHUTDOWN VOLTAGE (V)
Frequency Foldback
SWITCHING FREQUENCY (kHz) OR CURRENT (µA)
500
400
300
INPUT VOLTAGE (V)
FREQUENCY (kHz)
SWITCHING
FREQUENCY
200
100
FEEDBACK PIN
CURRENT
0
0
0.5
1.5
2.0
1.0
FEEDBACK PIN VOLTAGE (V)
2.5
4
U W
100
L11507 • TPC04
LT1507 • TPC07
LT1507 • TPC10
Standby and Shutdown Thresholds
30
Shutdown Supply Current
V
SHDN
= 0V
25
20
15
10
5
0
2.32
0.8
STARTUP
0.4
SHUTDOWN
125
0
50
100
25
75
–50 –25
0
JUNCTION TEMPERATURE (°C)
125
0
3
6
9
INPUT VOLTAGE (V)
12
15
LT1507 • TPC05
LT1507 • TPC06
Error Amplifier Transconductance
3000
Error Amplifier Transconductance
200
PHASE
2000
2500
GAIN
150
PHASE (DEG)
1500
2000
100
R
OUT
200k
V
C
C
OUT
12pF
1000
1500
V
FB
×
2e
–3
50
500
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
R
LOAD
= 50Ω
0.5
0
50
75 100
0
25
–50 –25
JUNCTION TEMPERATURE (°C)
125
500
100
1k
10k
100k
FREQUENCY (Hz)
1M
–50
10M
LT1507 • TPC08
LT1507 • TPC09
Switching Frequency
600
6.5
6.0
550
5.5
5.0
4.5
4.0
3.5
400
–50
3.0
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
125
Minimum Input Voltage
with 3.3V Output
MINIMUM VOLTAGE
TO START WITH
STANDARD CIRCUIT
500
450
MINIMUM VOLTAGE
TO RUN WITH
STANDARD CIRCUIT
1
10
100
LOAD CURRENT (mA)
1000
LT1507 • TPC12
LT1507 • TPC11
MINIMUM INPUT VOLTAGE CAN BE REDUCED
BY ADDING A SMALL EXTERNAL PNP. SEE
APPLICATIONS INFORMATION
LT1507
TYPICAL PERFORMANCE CHARACTERISTICS
Current Limit Foldback
2.5
*POSSIBLE
UNDESIRED
STABLE POINT
FOR CURRENT
SOURCE LOAD
FOLDBACK
CHARACTERISTICS
CURRENT
SOURCE LOAD
1.50
V
OUT
= 3.3V
1.25
L = 10µH
1.00
0.75
L = 3µH
0.50
0.5
MOS LOAD
0.25
RESISTOR LOAD
0
0
20
60
80
40
OUTPUT VOLTAGE (%)
100
0
4
6
8
10
INPUT VOLTAGE (V)
12
14
0
0
3
6
9
INPUT VOLTAGE (V)
12
15
L = 2µH
0.25
1.25
L = 10µH
1.00
L = 5µH
0.75
0.50
2.0
OUTPUT CURRENT (A)
CURRENT (A)
1.5
L = 5µH
1.0
LT1507 • TPC13
LT1507 • TPC14
CURRENT (A)
*SEE "MORE THAN JUST VOLTAGE FEEDBACK"
IN APPLICATIONS INFORMATION SECTION
Boost Pin Current
12
T
J
= 25°C
10
BOOST PIN CURRENT (mA)
CORE LOSS (W)
8
6
4
2
0
0.1
TYPE 52 POWDERED IRON
Kool Mµ
®
SWITCH VOLTAGE (V)
0
0.25
0.50
0.75
1.00
SWITCH CURRENT (A)
Kool Mµ is a registered trademark of Magnetics, Incorporated.
Metglas is a registered trademark of AlliedSignal Incorporated.
PIN FUNCTIONS
BOOST (Pin 1):
The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal
bipolar NPN power switch. Without this added voltage the
typical switch voltage loss would be about 1.5V. The
additional boost voltage allows the switch to saturate and
voltage loss approximates that of a 0.3Ω FET structure,
but with a much smaller die area. Efficiency improves from
70% for conventional bipolar designs to greater than 85%
for these new parts.
V
IN
(Pin 2):
Input Pin. The LT1507 is designed to operate
with an input voltage between 4.5V and 15V. Under certain
conditions, input voltage may be reduced down to 4V.
Actual minimum operating voltage will always be higher
than the output voltage. It may be limited by switch
U W
LT1507 • TPC16
Maximum Load Current
at V
OUT
= 3.3V
1.50
Maximum Load Current
at V
OUT
= 5V
L = 20µH
LT1507 • TPC15
Inductor Core Loss for 3.3V Output
1.0
V
OUT
= 3.3V
V
IN
= 5V
I
OUT
= 1A
0.6
0.8
Switch Voltage Drop
T
J
= 25°C
0.4
0.01
PERMALLOY
µ
= 125
Metglas
®
0.2
1.25
0.001
1
2
4
INDUCTANCE (µH)
6
8
10
0
0
0.25
0.50 0.75 1.00 1.25
SWITCH CURRENT (A)
1.50
LT1507 • TPC17
LT1507 • TPC18
CORE LOSS IS INDEPENDENT OF LOAD CURRENT
UNTIL LOAD CURRENT FALLS LOW ENOUGH
FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE
U
U
U
5
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