LT1375/LT1376
1.5A, 500kHz Step-Down
Switching Regulators
FEATURES
Constant 500kHz Switching Frequency
Uses All Surface Mount Components
Inductor Size Reduced to 5µH
Easily Synchronizable
Saturating Switch Design: 0.4Ω
Effective Supply Current: 2.5mA
Shutdown Current: 20µA
Cycle-by-Cycle Current Limiting
■
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■
■
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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 frequency. Efficiency is maintained over a wide output current 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 synchronized from 580kHz to 900kHz with logic level inputs.
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APPLICATIO S
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Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
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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
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-circuit protection and thermal shutdown are provided.
Standard surface mount external parts are used, including 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.
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TYPICAL APPLICATIO
5V Buck Converter
Efficiency vs Load Current
D2
1N914
C3*
10µF TO
50µF
C2
0.1µF
BOOST
+
LT1376-5 BIAS
DEFAULT
= ON
OUTPUT**
5V, 1.25A
VSW
VIN
SHDN
GND
L1**
5µH
FB
VC
CC
3.3nF
D2
1N5818
VOUT = 5V
VIN = 10V
L = 10µH
90
+
* RIPPLE CURRENT ≥ IOUT/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.
C1
100µF, 10V
SOLID
TANTALUM
EFFICIENCY (%)
INPUT
6V † TO 25V
100
80
70
60
50
0
1375/76 TA01
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
1375/76 TA02
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LT1375/LT1376
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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
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PACKAGE/ORDER INFORMATION
TOP VIEW
BOOST 1
VIN 2
VSW 3
SHDN 4
N8 PACKAGE
8-LEAD PDIP
TOP VIEW
8 VC
7 FB/SENSE
LT1375CN8
LT1375CN8-5
LT1375IN8
LT1375IN8-5
LT1375CS8
LT1375CS8-5
LT1375HVCS8
LT1375IS8
LT1375IS8-5
LT1375HVIS8
VIN 2
8 VC
7 FB/SENSE
6 GND
VSW 3
6 GND
5 SYNC
BIAS 4
5 SHDN
S8 PACKAGE
8-LEAD PLASTIC SO
θJA = 100°C/ W (N8)
θJA = 120°C/ W TO 150°C/W DEPENDING ON
PC BOARD LAYOUT (S8)
ORDER PART
NUMBER
BOOST 1
TOP VIEW
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
θJA = 100°C/ W (N8)
θJA = 120°C/ W TO 150°C/W DEPENDING ON
PC BOARD LAYOUT (S8)
S8 PART
MARKING
ORDER PART
NUMBER
1375
13755
1375HV
1375I
1375I5
375HVI
LT1376CN8
LT1376CN8-5
LT1376IN8
LT1376IN8-5
LT1376CS8
LT1376CS8-5
LT1376HVCS8
LT1376IS8
LT1376IS8-5
LT1376HVIS8
S8 PART
MARKING
GND 1
16 GND
NC 2
15 NC
BOOST 3
14 VC
VIN 4
13 FB/SENSE
VSW 5
12 GND
BIAS 6
11 SHDN
NC 7
GND 8
10 NC
9
GND
S PACKAGE
16-LEAD PLASTIC NARROW SO
θJA = 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
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.
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LT1375/LT1376
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open,
unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
2.39
2.36
2.42
●
2.45
2.48
V
V
4.94
4.90
5.0
●
5.06
5.10
V
V
7
10
14
kΩ
0.01
0.01
0.03
0.03
%/ V
%/V
0.5
1.5
µA
Reference Voltage (Adjustable)
Sense Voltage (Fixed 5V)
Sense Pin Resistance
Reference Voltage Line Regulation
5V ≤ VIN ≤ 25V
5V ≤ VIN ≤ 30V (LT1375HV/LT1376HV)
●
Feedback Input Bias Current
Error Amplifier Voltage Gain
VSHDN = 1V (Notes 2, 8)
Error Amplifier Transconductance
VSHDN = 1V, ∆I (VC) = ±10µA (Note 8)
●
200
400
1500
1100
2000
VC Pin to Switch Current Transconductance
Error Amplifier Source Current
2700
3000
2
●
VSHDN = 1V, VFB = 2.1V or VSENSE = 4.4V
150
225
µMho
µMho
A/ V
320
2
µA
Error Amplifier Sink Current
VSHDN = 1V, VFB = 2.7V or VSENSE = 5.6V
VC Pin Switching Threshold
Duty Cycle = 0
0.9
V
VC Pin High Clamp
VSHDN = 1V
2.1
V
Switch Current Limit
VBOOST = VIN + 5V
VC Open, VFB = 2.1V or VSENSE = 4.4V,
Switch On Resistance (Note 7)
ISW = 1.5A, VBOOST = VIN + 5V
DC ≤ 50%
DC = 80%
●
●
1.50
1.35
2
3
3
A
A
0.3
0.4
0.5
Ω
Ω
●
Maximum Switch Duty Cycle
VFB = 2.1V or VSENSE = 4.4V
Switch Frequency
VC Set to Give 50% Duty Cycle
●
86
93
500
●
460
440
440
540
560
570
kHz
kHz
kHz
0.05
0.05
0.15
0.15
%/ V
%/V
1.0
1.3
V
5.0
5.5
V
●
3
3.5
V
●
●
12
25
22
35
mA
mA
0°C ≤ TJ ≤ 125°C
Switch Frequency Line Regulation
5V ≤ VIN ≤ 25V
5V ≤ VIN ≤ 30V (LT1375HV/LT1376HV)
●
●
Frequency Shifting Threshold on FB Pin
∆f = 10kHz
●
0.8
●
Minimum Input Voltage (Note 3)
mA
%
Minimum Boost Voltage (Note 4)
ISW ≤ 1.5A
Boost Current (Note 5)
VBOOST = VIN + 5V
Input Supply Current (Note 6)
VBIAS = 5V
●
0.9
1.4
mA
Output Supply Current (Note 6)
VBIAS = 5V
●
3.2
4.0
mA
Shutdown Supply Current
VSHDN = 0V, VIN ≤ 25V, VSW = 0V, VC Open
15
50
75
µA
µA
20
75
100
µA
µA
2.38
2.46
V
ISW = 500mA
ISW = 1.5A
●
VSHDN = 0V, VIN ≤ 30V, VSW = 0V, VC Open
(LT1375HV/LT1376HV)
●
Lockout Threshold
VC Open
●
2.3
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LT1375/LT1376
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open,
unless otherwise noted.
PARAMETER
CONDITIONS
Shutdown Thresholds
VC Open
Device Shutting Down
Device Starting Up
VC Open
LT1375HV/LT1376HV Device Shutting Down
LT1375HV/LT1376HV Device Starting Up
Minimum Synchronizing Amplitude (LT1375 Only)
MIN
TYP
MAX
UNITS
●
●
0.15
0.25
0.37
0.45
0.60
0.60
V
V
●
●
0.15
0.25
0.37
0.45
0.70
0.70
V
V
●
VIN = 5V
1.5
Synchronizing Range (LT1375 Only)
580
SYNC Pin Input Resistance
2.2
V
900
kHz
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 VC 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
kΩ
5V. Total input referred supply current is calculated by summing input
supply current (ISI) with a fraction of output supply current (ISO):
ITOT = ISI + (ISO)(VOUT/VIN)(1.15)
With VIN = 15V, VOUT = 5V, ISI = 0.9mA, ISO = 3.6mA, ITOT = 2.28mA.
For the LT1375, quiescent current is equal to:
ITOT = ISI + ISO(1.15)
because the BIAS pin is internally connected to VIN.
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 VIN to VSW 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
VOUT/2.42.
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TYPICAL PERFORMANCE CHARACTERISTICS
Inductor Core Loss
0.4
PERMALLOY
µ = 125
0.01
0.2
0.12
0.08
CORE LOSS IS
INDEPENDENT OF LOAD
CURRENT UNTIL LOAD CURRENT FALLS
LOW ENOUGH FOR CIRCUIT TO GO INTO
DISCONTINUOUS MODE
0.02
5
10
15
INDUCTANCE (µH)
20
1.5
GUARANTEED MINIMUM
1.5
1.0
0.5
VOLTAGE
2.42
1.0
CURRENT
0.5
2.41
0.04
0.001
0
2.43
2.0
FEEDBACK VOLTAGE (V)
Kool Mµ®
2.0
CURRENT (µA)
2
1.2
0.8
2.44
TYPICAL
SWITCH PEAK CURRENT (A)
4
TYPE 52
POWDERED IRON
CORE LOSS (% OF 5W LOAD)
CORE LOSS (W)
VOUT = 5V, VIN = 10V, IOUT = 1A
0.1
Feedback Pin Voltage and Current
Switch Peak Current Limit
2.5
20
12
8
1.0
25
1375/76 G01
0
0
20
60
40
DUTY CYCLE (%)
80
100
1375/76 G08
2.40
–50
–25
0
25
50
75
100
JUNCTION TEMPERATURE (°C)
0
125
1375/76 G09
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LT1375/LT1376
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TYPICAL PERFORMANCE CHARACTERISTICS
Standby and Shutdown Thresholds
Shutdown Pin Bias Current
CURRENT REQUIRED TO FORCE SHUTDOWN
(FLOWS OUT OF PIN). AFTER SHUTDOWN,
CURRENT DROPS TO A FEW µA
VSHUTDOWN = 0V
300
200
8
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
4
INPUT SUPPLY CURRENT (µA)
STANDBY
SHUTDOWN PIN VOLTAGE (V)
CURRENT (µA)
400
2.36
2.32
0.8
START-UP
0.4
25
20
15
10
5
SHUTDOWN
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
0
50
100
25
75
–50 –25
0
JUNCTION TEMPERATURE (°C)
125
Shutdown Supply Current
VIN = 25V
75
VIN = 10V
25
10
15
INPUT VOLTAGE (V)
25
20
Error Amplifier Transconductance
2500
3000
2000
2500
200
PHASE
GAIN (µMho)
100
5
1500
1000
500
150
GAIN
2000
100
VC
(
)
ROUT
200k
COUT
12pF
1500
VFB 2 • 10–3
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
50
PHASE (DEG)
TRANSCONDUCTANCE (µMho)
125
0
1375/76 G06
Error Amplifier Transconductance
150
50
0
125
1375/76 G05
1375/76 G04
INPUT SUPPLY CURRENT (µA)
Shutdown Supply Current
30
2.40
500
0
RLOAD = 50Ω
0
0
0
50
25
0
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
0.5
0.1
0.2
0.3
0.4
SHUTDOWN VOLTAGE (V)
1k
Frequency Foldback
500
8.5
MINIMUM INPUT VOLTAGE CAN BE
REDUCED BY ADDING A SMALL EXTERNAL
PNP. SEE APPLICATIONS INFORMATION
8.0
400
FREQUENCY (kHz)
INPUT VOLTAGE (V)
550
200
500
450
100
FEEDBACK PIN
CURRENT
0
0.5
1.5
2.0
1.0
FEEDBACK PIN VOLTAGE (V)
7.5
MINIMUM
VOLTAGE TO
START WITH
STANDARD
CIRCUIT
7.0
6.5
6.0
MINIMUM VOLTAGE
TO RUN WITH
STANDARD CIRCUIT
5.5
0
2.5
1375/76 G10
400
–50
–50
10M
LT1376 Minimum Input Voltage
with 5V Output
600
300
1M
1375/76 G03
Switching Frequency
SWITCHING
FREQUENCY
10k
100k
FREQUENCY (Hz)
1375/76 G02
1375/76 G07
SWITCHING FREQUENCY (kHz) OR CURRENT (µA)
125
500
100
–25
0
25
50
75
100
JUNCTION TEMPERATURE (°C)
125
1375/76 G11
5.0
0
10
100
LOAD CURRENT (mA)
1000
1375/76 G12
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LT1375/LT1376
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TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Load Current
at VOUT = 5V
Maximum Load Current
at VOUT = 3.3V
Maximum Load Current
at VOUT = 10V
1.50
1.50
1.50
L = 20µH
VOUT = 10V
L = 20µH
1.25
1.25
L = 10µH
1.00
L = 5µH
L = 20µH
1.25
L = 10µH
0.75
L = 5µH
0.50
CURRENT (A)
1.00
CURRENT (A)
CURRENT (A)
L = 10µH
0.75
L = 5µH
0.75
0.50
0.50
0.25
1.00
0.25
0.25
VOUT = 5V
VOUT = 3.3V
0
0
5
10
15
INPUT VOLTAGE (V)
0
25
20
0
0
5
10
15
INPUT VOLTAGE (V)
1375/76 G13
BOOST Pin Current
4
2
0.25
0.50
0.75
1.00
SWITCH CURRENT (A)
1.25
TJ = 25°C
1.2
SWITCH VOLTAGE (V)
THRESHOLD VOLTAGE (V)
BOOST PIN CURRENT (mA)
10
25
20
0.8
SHUTDOWN
TJ = 25°C
6
10
15
INPUT VOLTAGE (V)
Switch Voltage Drop
VC Pin Shutdown Threshold
8
5
1375/76 G15
1.4
0
0
1375/76 G14
12
0
25
20
1.0
0.8
0.6
0.4
–50
–25
0
25
50
75
100
JUNCTION TEMPERATURE (°C)
1375/76 G16
125
1375/76 G11
0.6
0.4
0.2
0
0
0.25
0.50 0.75 1.00 1.25
SWITCH CURRENT (A)
1.50
1375/76 G18
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PIN FUNCTIONS
BOOST: 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
much smaller die area. Efficiency improves from 75% for
conventional bipolar designs to > 87% for these new parts.
VSW: The switch pin is the emitter of the on-chip power
NPN switch. It is driven up to the input pin voltage during
switch on time. Inductor current drives the switch pin
negative during switch off time. Negative voltage is clamped
with the external catch diode. Maximum negative switch
voltage allowed is – 0.8V.
SHDN: The shutdown pin is used to turn off the regulator
and to reduce input drain current to a few microamperes.
Actually, this pin has two separate thresholds, one at
2.38V to disable switching, and a second at 0.4V to force
complete micropower shutdown. The 2.38V threshold
functions as an accurate undervoltage lockout (UVLO).
This is sometimes used to prevent the regulator from
operating until the input voltage has reached a predetermined level.
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LT1375/LT1376
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PIN FUNCTIONS
VIN: This is the collector of the on-chip power NPN switch.
This pin powers the internal circuitry and internal regulator
when the BIAS pin is not present. At NPN switch on and off,
high dl/dt edges occur on this pin. Keep the external
bypass and catch diode close to this pin. All trace inductance on this path will create a voltage spike at switch off,
adding to the VCE voltage across the internal NPN.
BIAS (LT1376 Only): The BIAS pin is used to improve
efficiency when operating at higher input voltages and
light load current. Connecting this pin to the regulated
output voltage forces most of the internal circuitry to draw
its operating current from the output voltage rather than
the input supply. This is a much more efficient way of
doing business if the input voltage is much higher than the
output. Minimum output voltage setting for this mode of
operation is 3.3V. Efficiency improvement at VIN = 20V,
VOUT = 5V, and IOUT = 25mA is over 10%.
SYNC (LT1375 Only): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly
logic compatible and can be driven with any signal between 10% and 90% duty cycle. The synchronizing range
is equal to initial operating frequency, up to 900kHz. See
Synchronizing section in Applications Information for
details.
FB/SENSE: The feedback pin is used to set output voltage,
using an external voltage divider that generates 2.42V at
the pin with the desired output voltage. The fixed voltage
(-5) parts have the divider included on the chip, and the FB
pin is used as a SENSE pin, connected directly to the 5V
output. Two additional functions are performed by the FB
pin. When the pin voltage drops below 1.7V, switch
current limit is reduced. Below 1V, switching frequency is
also reduced. See Feedback Pin Function section in Applications Information for details.
VC: The VC pin is the output of the error amplifier and the
input of the peak switch current comparator. It is normally
used for frequency compensation, but can do double duty
as a current clamp or control loop override. This pin sits
at about 1V for very light loads and 2V at maximum load.
It can be driven to ground to shut off the regulator, but if
driven high, current must be limited to 4mA.
GND: The GND pin connection needs consideration for
two reasons. First, it acts as the reference for the regulated
output, so load regulation will suffer if the “ground” end of
the load is not at the same voltage as the GND pin of the
IC. This condition will occur when load current or other
currents flow through metal paths between the GND pin
and the load ground point. Keep the ground path short
between the GND pin and the load, and use a ground plane
when possible. The second consideration is EMI caused
by GND pin current spikes. Internal capacitance between
the VSW pin and the GND pin creates very narrow (
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