LT1576/LT1576-5
1.5A, 200kHz Step-Down
Switching Regulator
FEATURES
s
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s
s
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DESCRIPTIO
Constant 200kHz Switching Frequency
1.21V Reference Voltage
Fixed 5V Output Option
Easily Synchronizable
Uses All Surface Mount Components
Inductor Size Reduced to 15
µ
H
Saturating Switch Design: 0.2Ω
Effective Supply Current: 1.16mA
Shutdown Current: 20µA
Cycle-by-Cycle Current Limiting
Fused Lead SO-8 Package
The LT
®
1576 is a 200kHz monolithic buck mode switching
regulator. A 1.5A switch is included on the die along with
all the necessary oscillator, control and logic circuitry. The
topology is current mode for fast transient response and
good loop stability. The LT1576 is a modified version of the
industry standard LT1376 optimized for noise sensitive
applications.
In addition, the reference voltage has been lowered to
allow the device to produce output voltages down to 1.2V.
Quiescent current has been reduced by a factor of two.
Switch on resistance has been reduced by 30%. Switch tran-
sition times have been slowed to reduce EMI generation.
The oscillator frequency has been reduced to 200kHz to
maintain high efficiency over a wide output current range.
The pinout has been changed to improve PC layout by
allowing the high current high frequency switching cir-
cuitry to be easily isolated from low current noise sensitive
control circuitry. The new SO-8 package includes a fused
ground lead which significantly reduces the thermal resis-
tance of the device to extend the ambient operating tem-
perature range. There is an optional function of shutdown
or synchronization. Standard surface mount external parts
can be used including the inductor and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
TYPICAL APPLICATION
Efficiency vs Load Current
5V Buck Converter
INPUT
6V TO 25V
C3*
10µF TO
50µF
100
95
EFFICIENCY (%)
90
85
80
75
70
V
OUT
= 5V
V
IN
= 10V
L = 33µH
+
V
IN
BOOST
C2
0.33µF
V
SW
L1**
15µH
D2
1N914
OUTPUT**
5V, 1.25A
LT1576
OPEN = ON
SHDN
GND
V
C
BIAS
FB
D1
1N5818
C
C
100pF
R1
15.8k
R2
4.99k
* RIPPLE CURRENT RATING
≥
I
OUT
/2
** INCREASE L1 TO 30µH FOR LOAD
CURRENTS ABOVE 0.6A AND TO
60µH ABOVE 1A
SEE APPLICATIONS INFORMATION
+
C1
100µF, 10V
SOLID
TANTALUM
1576 TA01
U
0
0.25
0.50 0.75 1.00
LOAD CURRENT (A)
1.25
1.50
1576 TA02
U
U
1
LT1576/LT1576-5
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER
LT1576CS8
LT1576CS8-SYNC
LT1576IS8
LT1576IS8-SYNC
LT1576CS8-5
LT1576CS8-5 SYNC
LT1576IS8-5
LT1576IS8-5 SYNC
S8 PART MARKING
1576
1576SN
1576I
576ISN
15765
5765SN
1576I5
76I5SN
Input Voltage .......................................................... 25V
BOOST Pin Above Input Voltage ............................. 10V
SHDN Pin Voltage .................................................. 6.5V
BIAS Pin Voltage ...................................................... 7V
FB Pin Voltage (Adjustable Part) ............................ 3.5V
FB Pin Current (Adjustable Part) ............................ 1mA
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1576C ............................................... 0°C to 125° C
LT1576I ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
V
SW
1
V
IN
2
BOOST 3
GND 4
8 SHDN OR
SYNC*
7 FB OR SENSE*
6
5
V
C
BIAS
S8 PACKAGE
8-LEAD PLASTIC SO
θ
JA
= 80°C/ W WITH FUSED GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
*Default is the adjustable output voltage device with FB pin and shutdown
function. Option -5 replaces FB with SENSE pin for fixed 5V output
applications. -SYNC replaces SHDN with SYNC pin for applications
requiring synchronization. Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature
range, otherwise specifications are T
A
, T
J
= 25°C, V
IN
= 15V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Feedback Voltage
Sense Voltage (Fixed 5V)
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 (Note 9)
Switch On Resistance (Note 7)
Maximum Switch Duty Cycle
V
C
Open, V
FB
= 1.1V, DC
≤
50%
DC = 80%
I
SW
= 1.5A
q
q
q
CONDITIONS
5V
≤
V
IN
≤
25V, V
OL
+ 0.2V
≤
V
CC
≤
V
OH
– 0.2V
q
MIN
1.195
1.18
4.94
4.90
13
TYP
1.21
5.0
18.5
0.01
0.5
MAX
1.225
1.24
5.06
5.10
26
0.03
2
1300
1700
190
200
UNITS
V
V
V
V
kΩ
%/V
µA
µMho
µMho
A/ V
µA
µA
V
V
5V
≤
V
IN
≤
25V
q
(Notes 2, 8)
∆I
(V
C
) =
±10µA
(Note 8)
q
200
800
400
40
50
400
1050
1.5
V
FB
= 1V
V
FB
= 1.4V
Duty Cycle = 0
q
q
110
130
0.8
2.1
1.5
2
0.3
0.2
3.50
0.35
0.45
V
FB
= 1.1V
q
90
86
94
94
2
U
W
U
U
W W
W
A
A
Ω
Ω
%
%
LT1576/LT1576-5
range, otherwise specifications are T
A
, T
J
= 25°C, V
IN
= 15V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Minimum Switch Duty Cycle (Note 10)
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)
V
IN
Supply Current (Note 6)
BIAS Supply Current (Note 6)
Shutdown Supply Current
Lockout Threshold
Shutdown Thresholds
Synchronization Threshold
Synchronizing Range
SYNC Pin Input Resistance
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:
V
IN
supply current is the current drawn when the BIAS pin is held
at 5V and switching is disabled. Total input referred supply current is
calculated by summing input supply current (I
SI
) with a fraction of BIAS
supply current (I
SB
)
I
TOT
= I
SI
+ (I
SB
)(V
BIAS
/V
IN
)(1.15)
with V
IN
= 15V, V
BIAS
= 5V, I
SI
= 0.55mA, I
SB
= 1.6mA and I
TOT
= 1.16mA.
If the BIAS pin is unavailable or open circuit, the sum of V
IN
and BIAS
supply currents will be drawn by the V
IN
pin.
250
40
I
SW
≤
1.5A
I
SW
= 0.5A
I
SW
= 1.5A
V
BIAS
= 5V
V
BIAS
= 5V
V
SHDN
= 0V, V
IN
≤
25V, V
SW
= 0V, V
C
Open
q
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature
CONDITIONS
V
C
Set to Give 50% Duty Cycle
q
MIN
180
160
0.4
TYP
8
200
0
0.74
5.0
2.3
9
27
0.55
1.6
20
MAX
220
240
0.15
1.0
5.5
3.0
18
50
0.8
2.2
50
75
2.50
0.60
0.7
2.2
400
UNITS
%
kHz
kHz
%/ V
V
V
V
mA
mA
mA
mA
µA
µA
V
V
V
V
kHz
kΩ
5V
≤
V
IN
≤
25V
∆f
= 10kHz
q
q
q
q
q
q
q
q
V
C
Open
V
C
Open Device Shutting Down
Device Starting Up
q
q
q
2.34
0.13
0.25
2.42
0.37
0.45
1.5
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 the SENSE pin on the fixed voltage parts. Divide values shown by
the ratio V
OUT
/1.21.
Note 9:
Slope compensation is the current subtracted from the switch
current limit at 80% duty cycle. See Maximum Output Load Current in the
Applications Information section for further details.
Note 10:
Minimum on-time is 400ns typical. For a 200kHz operating
frequency this means the minimum duty cycle is 8%. In frequency
foldback mode, the effective duty cycle will be less than 8%.
3
LT1576/LT1576-5
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Drop
0.5
125°C
2.5
TYPICAL
SWITCH PEAK CURRENT (A)
0.4
SWITCH VOLTAGE (V)
25°C
0.3
1.5
MINIMUM
–20°C
0.2
FEEDBACK VOLTAGE (V)
0.1
0
0
0.25
0.50 0.75 1.00
SWITCH CURRENT (A)
1.25
1.50
Shutdown Pin Bias Current
4
AT 2.44V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
3
140
SHUTDOWN PIN VOLTAGE (V)
CURRENT (µA)
CURRENT (µA)
2
1
0
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
Standby Thresholds
2.46
2.45
2.44
ON
2.43
STANDBY
2.42
2.41
2.40
50
100
–50 –25
25
75
0
JUNCTION TEMPERATURE (°C)
25
INPUT SUPPLY CURRENT (µA)
SHUTDOWN PIN VOLTAGE (V)
GAIN (µMho)
4
U W
1576 G01
1576 G04
Switch Peak Current Limit
1.23
Feedback Pin Voltage
2.0
1.22
1.21
1.0
0.5
1.20
0
0
20
60
40
DUTY CYCLE (%)
80
100
1576 G02
1.19
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
1576 G03
Shutdown Pin Bias Current
180
160
0.8
0.7
0.6
0.5
0.4
Shutdown Thresholds
120
100
80
60
40
20
CURRENT REQUIRED TO FORCE
SHUTDOWN (FLOWS OUT OF PIN).
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
µA
125
START-UP
SHUTDOWN
0.3
0.2
0.1
0
0
50
25
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
125
0
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
1576 G05
1576 G06
Shutdown Supply Current
2000
V
SHDN
= 0V
Error Amplifier Transconductance
200
PHASE
20
1500
GAIN
150
PHASE (DEG)
15
1000
100
10
500
V
FB
1
×
10
–3
(
)
R
OUT
570k
V
C
C
OUT
2.4pF
50
5
0
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
R
LOAD
= 50Ω
0
–500
0
5
15
10
INPUT VOLTAGE (V)
20
25
1576 G08
125
10
100
1k
10k
FREQUENCY (Hz)
100k
–50
1M
1576 G09
1576 G07
LT1576/LT1576-5
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Supply Current
100
1600
1400
INPUT SUPPLY CURRENT (µA)
TRANSCONDUCTANCE (µMho)
SWITCHING FREQUENCY (kHz)
OR CURRENT (µA)
75
V
IN
= 25V
V
IN
= 10V
50
25
0
0
0.2
0.3
0.1
SHUTDOWN VOLTAGE (V)
Switching Frequency
240
220
INPUT VOLTAGE (V)
FREQUENCY (kHz)
CURRENT (A)
200
180
160
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
Maximum Load Current
at V
OUT
= 5V
1.50
1.25
1.00
0.75
0.50
0.25
V
OUT
= 5V
0
0
1.50
L = 60µH
L = 30µH
CURRENT (A)
CURRENT (A)
L = 15µH
CORE LOSS (W)
L = 15µH
0
5
10
15
INPUT VOLTAGE (V)
Kool Mµ is a registered trademark of Magnetics, Inc.
U W
1576 G10
1576 G13
Error Amplifier Transconductance
250
Frequency Foldback
SWITCHING FREQUENCY
200
1200
1000
800
600
400
200
150
100
50
FEEDBACK PIN CURRENT
0.4
0
0
50
25
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
0
0
1.0
0.5
1.5
FEEDBACK VOLTAGE (V)
2.0
1576 G12
1576 G11
Minimum Input Voltage
at V
OUT
= 5V
7
Maximum Load Current
at V
OUT
= 10V
1.50
V
OUT
= 10V
1.25
L = 30µH
1.00
0.75
0.50
0.25
L = 15µH
L = 60µH
V
OUT
= 5V
MINIMUM
STARTING VOLTAGE
6
MINIMUM
RUNNING VOLTAGE
125
5
0
1
100
10
LOAD CURRENT (mA)
1000
1576 G14
0
5
10
15
INPUT VOLTAGE (V)
20
25
1576 G15
Maximum Load Current
at V
OUT
= 3.3V
1.0
1.25
1.00
0.75
0.50
0.25
V
OUT
= 3.3V
0.001
Inductor Core Loss
V
OUT
= 5V, V
IN
= 10V, I
OUT
= 1A
20
12
8
L = 60µH
L = 30µH
0.1
TYPE 52
POWDERED IRON
Kool Mµ
®
PERMALLOY
µ
= 125
CORE LOSS (% OF 5W LOAD)
4
2
1.2
0.8
0.4
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
0.2
0.12
0.08
0.04
0.02
20
25
1576 G16
0
5
10
15
INPUT VOLTAGE (V)
20
25
1576 G17
1576 G18
5
