LT1375/LT1376
1.5A, 500kHz Step-Down
Switching Regulators
FEATURES
■
■
■
■
■
■
■
■
Constant 500kHz Switching Frequency
Uses All Surface Mount Components
Inductor Size Reduced to 5
µ
H
Easily Synchronizable
Saturating Switch Design: 0.4Ω
Effective Supply Current: 2.5mA
Shutdown Current: 20µA
Cycle-by-Cycle Current Limiting
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
techniques achieve high efficiency at high switching fre-
quency. Efficiency is maintained over a wide output cur-
rent range by using the output to bias the circuitry and by
utilizing a supply boost capacitor to saturate the power
switch. A shutdown signal will reduce supply current to
20µA on both parts. The LT1375 can be externally syn-
chronized from 580kHz to 900kHz with logic level inputs.
The LT1375/LT1376 fit into standard 8-pin PDIP and SO
packages, as well as a fused lead 16-pin SO with much
lower thermal resistance. Full cycle-by-cycle short-cir-
cuit protection and thermal shutdown are provided.
Standard surface mount external parts are used, includ-
ing the inductor and capacitors.
For low input voltage applications with 3.3V output, see
LT1507. This is a functionally identical part that can
operate with input voltages between 4.5V and 12V.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
DESCRIPTIO
The LT
®
1375/LT1376 are 500kHz monolithic buck mode
switching regulators. A 1.5A 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
TYPICAL APPLICATIO
5V Buck Converter
D2
1N914
C2
0.1µF
V
SW
LT1376-5 BIAS
L1**
5µH
6V
†
TO 25V
C3*
10µF TO
50µF
+
DEFAULT
= ON
V
IN
EFFICIENCY (%)
INPUT
BOOST
SHDN
GND
* RIPPLE CURRENT
≥
I
OUT
/2
** INCREASE L1 TO 10µH FOR LOAD CURRENTS ABOVE 0.6A AND TO 20µH ABOVE 1A
†
FOR INPUT VOLTAGE BELOW 7.5V, SOME RESTRICTIONS MAY APPLY.
SEE APPLICATIONS INFORMATION.
U
U
U
Efficiency vs Load Current
100
V
OUT
= 5V
V
IN
= 10V
L = 10µH
90
OUTPUT**
5V, 1.25A
80
FB
V
C
C
C
3.3nF
D2
1N5818
+
C1
100µF, 10V
SOLID
TANTALUM
70
60
50
0
1375/76 TA01
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
1375/76 TA02
13756fd
1
LT1375/LT1376
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
Input Voltage
LT1375/LT1376 .................................................. 25V
LT1375HV/LT1376HV ........................................ 30V
BOOST Pin Voltage
LT1375/LT1376 .................................................. 35V
LT1375HV/LT1376HV ........................................ 40V
SHDN Pin Voltage ..................................................... 7V
BIAS Pin Voltage ...................................................... 7V
FB Pin Voltage (Adjustable Part) ............................ 3.5V
FB Pin Current (Adjustable Part) ............................ 1mA
Sense Voltage (Fixed 5V Part) .................................. 7V
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1375C/LT1376C ............................... 0°C to 125° C
LT1375I/LT1376I ............................. – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
PACKAGE/ORDER INFORMATION
TOP VIEW
BOOST
1
V
IN
2
V
SW
3
SHDN
4
N8 PACKAGE
8-LEAD PDIP
TOP VIEW
8
V
C
7
FB/SENSE
6
GND
5
SYNC
S8 PACKAGE
8-LEAD PLASTIC SO
BOOST 1
V
IN
2
V
SW
3
BIAS 4
N8 PACKAGE
8-LEAD PDIP
θ
JA
= 100°C/ W (N8)
θ
JA
= 120°C/ W TO 150°C/W DEPENDING ON
PC BOARD LAYOUT (S8)
θ
JA
= 100°C/ W (N8)
θ
JA
= 120°C/ W TO 150°C/W DEPENDING ON
PC BOARD LAYOUT (S8)
ORDER PART
NUMBER
LT1375CN8
LT1375CN8-5
LT1375IN8
LT1375IN8-5
LT1375CS8
LT1375CS8-5
LT1375HVCS8
LT1375IS8
LT1375IS8-5
LT1375HVIS8
S8 PART
MARKING
ORDER PART
NUMBER
LT1376CN8
LT1376CN8-5
LT1376IN8
LT1376IN8-5
LT1376CS8
LT1376CS8-5
LT1376HVCS8
LT1376IS8
LT1376IS8-5
LT1376HVIS8
1375
13755
1375HV
1375I
1375I5
375HVI
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
2
U
U
W
W W
U
W
TOP VIEW
8 V
C
7 FB/SENSE
6 GND
5 SHDN
GND 1
NC 2
BOOST 3
V
IN
4
V
SW
5
BIAS 6
NC 7
GND 8
16 GND
15 NC
14 V
C
13 FB/SENSE
12 GND
11 SHDN
10 NC
9
GND
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART
MARKING
S PACKAGE
16-LEAD PLASTIC NARROW SO
θ
JA
= 50°C/ W WITH FUSED CORNER PINS
CONNECTED TO GROUND PLANE OR LARGE
LANDS
ORDER PART NUMBER
1376
13765
1376HV
1376I
1376I5
376HVI
LT1376CS
LT1376IS
LT1376HVCS
LT1376HVIS
13756fd
LT1375/LT1376
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. T
J
= 25°C, V
IN
= 15V, V
C
= 1.5V, boost open, switch open,
unless otherwise noted.
PARAMETER
Reference Voltage (Adjustable)
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
2.39
2.36
4.94
4.90
7
TYP
2.42
5.0
10
0.01
0.01
MAX
2.45
2.48
5.06
5.10
14
0.03
0.03
1.5
2700
3000
320
UNITS
V
V
V
V
kΩ
%/ V
%/V
µA
µMho
µMho
A/ V
µA
mA
V
V
Sense Voltage (Fixed 5V)
●
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
V
BOOST
= V
IN
+ 5V
Switch On Resistance (Note 7)
Maximum Switch Duty Cycle
Switch Frequency
V
SHDN
= 1V, V
FB
= 2.1V or V
SENSE
= 4.4V
V
SHDN
= 1V, V
FB
= 2.7V or V
SENSE
= 5.6V
Duty Cycle = 0
V
SHDN
= 1V
V
C
Open, V
FB
= 2.1V or V
SENSE
= 4.4V,
DC
≤
50%
DC = 80%
●
●
●
●
5V
≤
V
IN
≤
25V
5V
≤
V
IN
≤
30V (LT1375HV/LT1376HV)
●
0.5
200
400
2000
2
150
225
2
0.9
2.1
1.50
1.35
2
0.3
1500
1100
V
SHDN
= 1V (Notes 2, 8)
V
SHDN
= 1V,
∆I
(V
C
) =
±10µA
(Note 8)
●
3
3
0.4
0.5
540
560
570
0.15
0.15
1.3
5.5
3.5
22
35
1.4
4.0
50
75
75
100
2.46
A
A
Ω
Ω
%
kHz
kHz
kHz
%/ V
%/V
V
V
V
mA
mA
mA
mA
µA
µA
µA
µA
V
I
SW
= 1.5A, V
BOOST
= V
IN
+ 5V
V
FB
= 2.1V or V
SENSE
= 4.4V
V
C
Set to Give 50% Duty Cycle
0°C
≤
T
J
≤
125°C
●
●
●
●
●
●
●
86
460
440
440
93
500
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)
Output Supply Current (Note 6)
Shutdown Supply Current
5V
≤
V
IN
≤
25V
5V
≤
V
IN
≤
30V (LT1375HV/LT1376HV)
∆f
= 10kHz
I
SW
≤
1.5A
V
BOOST
= V
IN
+ 5V
V
BIAS
= 5V
V
BIAS
= 5V
V
SHDN
= 0V, V
IN
≤
25V, V
SW
= 0V, V
C
Open
I
SW
= 500mA
I
SW
= 1.5A
0.05
0.05
0.8
1.0
5.0
3
12
25
0.9
3.2
15
●
●
●
●
●
V
SHDN
= 0V, V
IN
≤
30V, V
SW
= 0V, V
C
Open
(LT1375HV/LT1376HV)
●
20
●
Lockout Threshold
V
C
Open
2.3
2.38
13756fd
3
LT1375/LT1376
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. T
J
= 25°C, V
IN
= 15V, V
C
= 1.5V, boost open, switch open,
unless otherwise noted.
PARAMETER
Shutdown Thresholds
CONDITIONS
V
C
Open
V
C
Open
Device Shutting Down
Device Starting Up
LT1375HV/LT1376HV Device Shutting Down
LT1375HV/LT1376HV Device Starting Up
●
●
●
●
●
ELECTRICAL CHARACTERISTICS
MIN
0.15
0.25
0.15
0.25
TYP
0.37
0.45
0.37
0.45
1.5
MAX
0.60
0.60
0.70
0.70
2.2
900
UNITS
V
V
V
V
V
kHz
kΩ
Minimum Synchronizing Amplitude (LT1375 Only)
Synchronizing Range (LT1375 Only)
SYNC Pin Input Resistance
V
IN
= 5V
580
40
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
Gain is measured with a V
C
swing equal to 200mV above the low
clamp 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
when the BIAS pin is held at 5V with switching disabled. Output supply
current is the current drawn by the BIAS pin when the bias pin is held at
5V. Total input referred supply current is calculated by summing input
supply current (I
SI
) with a fraction of output supply current (I
SO
):
I
TOT
= I
SI
+ (I
SO
)(V
OUT
/V
IN
)(1.15)
With V
IN
= 15V, V
OUT
= 5V, I
SI
= 0.9mA, I
SO
= 3.6mA, I
TOT
= 2.28mA.
For the LT1375, quiescent current is equal to:
I
TOT
= I
SI
+ I
SO
(1.15)
because the BIAS pin is internally connected to V
IN
.
For LT1375 or BIAS open circuit, input supply current is the sum of input
+ output supply currents.
Note 7:
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 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
Inductor Core Loss
1.0
V
OUT
= 5V, V
IN
= 10V, I
OUT
= 1A
TYPE 52
POWDERED IRON
Kool Mµ
®
CORE LOSS (W)
0.1
2
1.2
0.8
0.4
FEEDBACK VOLTAGE (V)
4
SWITCH PEAK CURRENT (A)
0.01
CORE LOSS IS
INDEPENDENT OF LOAD
CURRENT UNTIL LOAD CURRENT FALLS
LOW ENOUGH FOR CIRCUIT TO GO INTO
DISCONTINUOUS MODE
0
5
10
15
INDUCTANCE (µH)
20
25
PERMALLOY
µ
= 125
0.001
4
U W
1375/76 G01
Switch Peak Current Limit
20
12
8
CORE LOSS (% OF 5W LOAD)
2.5
TYPICAL
2.0
Feedback Pin Voltage and Current
2.44
2.0
2.43
1.5
CURRENT (µA)
1.5
GUARANTEED MINIMUM
2.42
VOLTAGE
1.0
1.0
0.2
0.12
0.08
0.04
0.02
CURRENT
2.41
0.5
0.5
0
0
20
60
40
DUTY CYCLE (%)
80
100
1375/76 G08
2.40
–50
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
0
125
1375/76 G09
13756fd
LT1375/LT1376
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Pin Bias Current
500
400
300
200
8
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
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)
50
25
75
0
TEMPERATURE (°C)
Shutdown Supply Current
150
125
100
V
IN
= 25V
75
50
25
0
V
IN
= 10V
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
INPUT VOLTAGE (V)
FREQUENCY (kHz)
SWITCHING
FREQUENCY
300
200
100
FEEDBACK PIN
CURRENT
0
0
0.5
1.5
2.0
1.0
FEEDBACK PIN VOLTAGE (V)
2.5
1375/76 G10
U W
100
1375/76 G04
Standby and Shutdown Thresholds
30
Shutdown Supply Current
V
SHUTDOWN
= 0V
25
20
15
10
5
0
2.32
0.8
START-UP
0.4
SHUTDOWN
125
0
50
100
25
75
–50 –25
0
JUNCTION TEMPERATURE (°C)
125
0
5
10
15
INPUT VOLTAGE (V)
20
25
1375/76 G06
1375/76 G05
Error Amplifier Transconductance
2500
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
0.5
1375/76 G07
0
50
25
0
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
500
100
1k
10k
100k
FREQUENCY (Hz)
1M
–50
10M
1375/76 G03
1375/76 G02
Switching Frequency
600
8.5
8.0
550
7.5
7.0
6.5
6.0
5.5
400
–50
5.0
LT1376 Minimum Input Voltage
with 5V Output
MINIMUM INPUT VOLTAGE CAN BE
REDUCED BY ADDING A SMALL EXTERNAL
PNP. SEE APPLICATIONS INFORMATION
500
MINIMUM
VOLTAGE TO
START WITH
STANDARD
CIRCUIT
MINIMUM VOLTAGE
TO RUN WITH
STANDARD CIRCUIT
0
10
100
LOAD CURRENT (mA)
1000
1375/76 G12
450
–25
100
JUNCTION TEMPERATURE (°C)
0
25
50
75
125
1375/76 G11
13756fd
5
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