LT1176/LT1176-5
Step-Down
Switching Regulator
FEATURES
s
s
s
s
s
s
s
s
1.2A On-Board Switch
100kHz Switching Frequency
Excellent Dynamic Behavior
DIP and Surface Mount Packages
Only 8mA Quiescent Current
Preset 5V Output Available
Operates Up to 35V Input
Micropower Shutdown Mode
tion. The power switch, all oscillator and control circuitry,
and all current limit components are included on the chip.
The topology is a classic positive “buck” configuration but
several design innovations allows this device to be used as
a positive-to-negative converter, a negative boost con-
verter, and as a flyback converter. The switch output is
specified to swing below ground.
The LT1176 uses a true analog multiplier in the feedback
loop. This makes the device respond nearly instantaneously
to input voltage fluctuations and makes loop gain indepen-
dent of input voltage. As a result, dynamic behavior of the
regulator is significantly improved over previous designs.
On-chip pulse by pulse current limiting makes the LT1176
nearly bust-proof for output overloads or shorts. The input
voltage range as a buck converter is 8V to 35V, but a self-
boot feature allows input voltages as low as 5V in the
inverting and boost configurations.
The LT1176 is available in a low cost 8-lead DIP package
with frequency preset at 100kHz and current limit at 1.7A An
adjustable output is offered as well as a preset 5V version.
For further design details and application help, see the
LT1074/LT1076 data sheet and Application Note 44.
APPLICATI
s
s
s
s
Buck Converter with Output Voltage Range of
2.5V to 30V
Positive-to-Negative Converter
Negative Boost Converter
Mulitple Output Buck Converter
DESCRIPTIO
The LT1176 is a 1A monolithic bipolar switching regulator
which requires only a few external parts for normal opera-
TYPICAL APPLICATI
Basic 5V Positive Buck Converter
L1*
100µH
10V T0 35V
V
IN
LT1176-5
SENSE
V
C
R3
2.7k
C3
100µF
C2
0.01µF
V
SW
1N5819
EFFICIENCY (%)
5V, 0.8A
GND
+
*COILTRONICS #100-1-52
LT1176/76-5 • TA01
PULSE ENGINEERING, INC. #PE-92102
HURRICANE #HL-AG210LL
THESE ARE LOW COST POWDERED IRON CORES. OPTIMUM EFFICIENCY
AND SMALLEST SIZE IS OBTAINED BY USING A LOW LOSS CORE SUCH
AS MAGNETICS INC. KOOL Mµ. SEE EFFICIENCY GRAPH.
UO
U
UO
S
5V Buck Converter Efficiency
90
L = 100µH ON MAGNETICS INC.
SIZE T-50 KOOL Mµ CORE
V
IN
= 10V
80
V
IN
= 20V
+
C1
470µF
25V
70
60
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
LT1176/76-5 • TA02
1
LT1176/LT1176-5
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
IN
1
I
LIM
2
GND 3
FB/SENSE 4
8
7
6
5
V
SW
V
C
STATUS
SHUTDOWN
Input Voltage .......................................................... 38V
Switch Voltage With Respect to Input Voltage ........ 50V
Switch Voltage With Respect to Ground Pin
(V
SW
Negative) (Note 6) ......................................... 20V
Feedback Pin Voltage....................................... – 2V, 10V
Shutdown Pin Voltage (Not to Exceed V
IN
) ............. 35V
Status Pin Voltage
(Current Must Be Limited to 5mA When Status Pin
Switches “ON”) ................................................... 30V
I
LIM
Pin Voltage (Forced)...................................... 5.5V
Maximum Operating Ambient Temperature Range
LT1176C/LT1176C-5 .............................. 0°C to 70°C
Maximum Operating Junction Temperature Range
LT1176C/LT1176C-5 ............................ 0°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec) 300°C
*These thermal resistance numbers are for typical mounting technique. Lower thermal resistance
can be obtained with large copper lands, thermal glues or heatsinks.
ORDER PART
NUMBER
LT1176CN8
LT1176CN8-5
N8 PACKAGE
8-LEAD PLASTIC DIP
T
J MAX
= 125°C,
θ
JA
= 90°C/ W*
TOP VIEW
V
IN
1
NC 2
I
LIM
3
GND 4
GND 5
GND 6
GND 7
NC 8
FB/SENSE 9
NC 10
20 V
SW
19 NC
18 V
C
17 GND
16 GND
15 GND
14 GND
13 STATUS
12 SHUTDOWN
11 NC
S PACKAGE
20-LEAD PLASTIC SOL
T
JMAX
= 125
°C, θ
JA
= 50°C/ W*
ORDER PART
NUMBER
LT1176CS
LT1176CS-5
ELECTRICAL CHARACTERISTICS
PARAMETER
Switch “ON” Voltage (Note 1)
Switch “OFF” Leakage
Supply Current (Note 2)
Minimum Supply Voltage
Switch Current Limit (Note 4)
CONDITIONS
I
SW
= 0.2A
I
SW
= 1A
T
J
= 25°C, V
IN
= 25V, unless otherwise noted
MIN
q
q
TYP
MAX
1.1
1.4
150
250
UNITS
V
V
µA
µA
mA
µA
V
V
A
A
A
%
kHz
kHz
kHz
kHz
%/ V
V/ V
µmho
µA
mA
V
IN
= 25V, V
SW
= 0
V
IN
= V
MAX
, V
SW
= 0 (Note 7)
V
FB
= 2.5V, V
IN
≤
35V
V
SHDN
= 0.1V (Device Shutdown) (Note 8)
Normal Mode
Startup Mode (Note 3)
I
LIM
Open
R
LIM
= 10k (Note 5)
R
LIM
= 7k (Note 5)
q
q
q
q
q
7.5
140.0
7.3
3.5
1.2
1.7
1.2
0.8
90
100
20
20
10
300
8.0
4.8
2.2
Maximum Duty Cycle
Switching Frequency
q
q
85
90
85
110
120
V
FB
= 0V Through 2k (Note 4) (LT1176)
V
SENSE
= 0V (Note 4) (LT1176-5)
Switching Frequency Line Regulation
Error Amplifier Voltage Gain (Note 9)
Error Amplifier Transconductance (Note 9)
Error Amplifier Source and Sink Current
Source (V
FB
= 2V or V
SENSE
= 4V)
Sink (V
FB
= 2.5V or V
SENSE
= 5.5V)
8V
≤
V
IN
≤
V
MAX
(Note 7)
1V
≤
V
C
≤
4V
3700
100.0
0.7
q
0.03
2000
5000
140.0
1.0
0.1
8000
225.0
1.6
2
U
W
U
U
W W
W
LT1176/LT1176-5
ELECTRICAL CHARACTERISTICS
PARAMETER
Feedback Pin Bias Current (LT1176)
Reference Voltage (LT1176)
Reference Voltage Tolerance (LT1176)
CONDITIONS
V
FB
= V
REF
V
C
= 2V
T
J
= 25°C, V
IN
= 25V, unless otherwise noted
MIN
q
q
q
q
q
TYP
0.5
2.21
±0.5
±1.0
MAX
2
2.265
±1.5
±2.5
5.15
±2
±3
8
0.02
UNITS
µA
V
%
%
V
%
%
kΩ
%/ V
V
mV/°C
V
2.155
V
REF
(Nominal) = 2.21V
All Conditions of Input Voltage, Output
Voltage, Temperature and Load Current
V
C
= 2V
V
OUT
(Nominal) = 5V
All Conditions of Input Voltage,
Temperature and Load Current
8V
≤
V
IN
≤
V
MAX
(Note 7)
Over Temperature
Sense Voltage (LT1176-5)
Sense Voltage Tolerance (LT1176-5)
4.85
5
±0.5
±1.0
Sense Pin Divider Resistance (LT1176-5)
Output Voltage Line Regulation
V
C
Voltage at 0% Duty Cycle
q
q
3
5
0.005
1.5
– 4.0
24
Multiplier Reference Voltage
Shutdown Pin Current
V
SHDN
≤
V
THRESHOLD
(≅ 2.5V)
Shutdown Thresholds
Status Window
Status High Level
Status Low Level
Status Delay Time
Status Minimum Width
The
q
denotes specifications which apply over the full operating tempera-
ture range.
Note 1:
To calculate maximum switch “ON” voltage at current between low
and high conditions, a linear interpolation may be used.
Note 2:
A feedback pin voltage (V
FB
) of 2.5V forces the V
C
pin to its low
clamp level and the switch duty cycle to zero. This approximates the zero
load condition where duty cycle approaches zero. The LT1176-5 has
V
SENSE
= 5.5V.
Note 3:
Total voltage from V
IN
pin to ground pin must be
≥8V
after start-
up for proper regulation.
Note 4:
Switch frequency is internally scaled down when the feedback pin
Application Hints
Although the LT1176 has a peak switch rating of 1.2A and a maximum duty
cycle of 85%, it must be used cautiously in applications which require high
switch current and high duty cycle simultaneously, to avoid excessive chip
temperature. Thermal resistance is 90°C/W for the 8-pin DIP package and
50°C/W for the 20-pin SO. This limits continuous chip power dissipation to
the 0.5W to 1W range. These numbers assume typical mounting techniques.
Extra or thick copper connected to the leads can reduce thermal resistance.
Bonding the package to the board or using a clip style heatsink can also help.
The following formulas will give chip power dissipation and peak switch
current for the standard buck converter. Note that surges less than 30
seconds do not need to be considered from a thermal standpoint, but for
proper regulation, they must not result in peak switch currents exceeding the
1.2A limit.
V
SHDN
= 5V
q
q
5
q
q
10
50
2.2
0.1
±4
3.5
20
µA
2.45
0.30
±5
4.5
0.25
9
30
µA
2.7
0.5
±6
5.0
0.4
V
V
%
V
V
µs
µs
Switch Duty Cycle = 0
Fully Shut Down
As a Percent of Output Voltage
I
STATUS
= 10µA Sourcing
I
STATUS
= 1.6mA Sinking
q
q
voltage is less than 1.3V to avoid extremely short switch-on times. During
testing, V
FB
or V
SENSE
is adjusted to give a minimum switch-on time of 1µs.
Note 5:
I
LM
= (R
LIM
– 1k)/ 7.65k
Note 6:
Switch to input voltage limitation must also be observed.
Note 7:
V
MAX
= 35V
Note 8:
Does not include switch leakage.
Note 9:
Error amplifier voltage gain and transconductance are specified
relative to the internal feedback node. To calculate gain and transconduc-
tance from the sense pin (Output) to the V
C
pin on the LT1176-5, multiply
by 0.44.
Power = I
LOAD
(V
OUT
/V
IN
) + V
IN
[7mA + 3mA (V
OUT
/V
IN
) + 0.012 (I
LOAD
)]
I
PEAK
= I
LOAD(PEAK)
+ [V
OUT
(V
IN
– V
OUT
)]/2E
5
(V
IN
)(L)
Example: V
IN
= 15V, V
OUT
= 5V, I
LOAD
= 0.5A Continuous, 0.8A Peak,
L = 100µH
Power (I
LOAD
= 0.5A) = 0.38W
I
PEAK
(I
LOAD
= 0.8A) = 0.97A
Where component size or height is critical, we suggest using solid
tantalum capacitors (singly or in parallel), but be sure to use units rated
for switching applications. Coiltronics is a good source for low profile
surface mount inductors and AVX makes high quality surface mount
tantalum capacitors. For further help, use Application Notes 19 and 44,
LTC’s SwitcherCAD computer design program, and our knowledgeable
application department.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
3
LT1176/LT1176-5
BLOCK DIAGRA
INPUT SUPPLY
10µA
0.3V
µPOWER
SHUTDOWN
2.35V
SHUTDOWN
OUTPUT
VOLTAGE
MONITOR
STATUS
2.21V
2.8k
FB/SENSE
*CONNECTED ON 5V VERSION ONLY
PACKAGE DESCRIPTIO
0.300 – 0.320
(7.620 – 8.128)
0.009 – 0.015
(0.229 – 0.381)
0.065
(1.651)
TYP
0.125
(3.175)
MIN
0.020
(0.508)
MIN
(
+0.025
0.325 –0.015
8.255
+0.635
–0.381
)
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
0.291 – 0.299
(7.391 – 7.595)
0.005
(0.127)
RAD MIN
0.010 – 0.029
×
45°
(0.254 – 0.737)
0.093 – 0.104
(2.362 – 2.642)
0.037 – 0.045
(0.940 – 1.143)
20
19
18
0° – 8° TYP
0.050
(1.270)
TYP
0.014 – 0.019
(0.356 – 0.482)
TYP
0.394 – 0.419
(10.007 – 10.643)
0.009 – 0.013
(0.229 – 0.330)
SEE NOTE
0.016 – 0.050
(0.406 – 1.270)
NOTE:
PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS.
1
2
3
4
5
6
7
8
9
10
4
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
q
FAX
: (408) 434-0507
q
TELEX
: 499-3977
U
W
+
320µA
6V
REGULATOR
AND BIAS
6V TO ALL
CIRCUITRY
–
CURRENT LIMIT
SHUTDOWN
500Ω
+
–
CURRENT LIMIT
COMP
C2
I
LIM
4.5V
10k
+
–
0.16Ω
FREQ SHIFT
100kHz
OSCILLATOR
SYNC
V
IN
Z
ANALOG
X MULTIPLIER
XYZ
Y
V
C
24V (EQUIVALENT)
3V
P-P
S
R
R/S
Q
LATCH
R
G1
+
C1
400Ω
+
–
A1
ERROR
AMP
–
PULSE WIDTH
COMPARATOR
SWITCH
OUTPUT
(V
SW
)
LT1176/76-5 • BD
2.2k*
Dimensions in inches (milimeters) unless otherwise noted.
N Package 8-Lead Plastic DIP
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
8
0.400
(10.160)
MAX
7
6
5
0.250 ± 0.010
(6.350 ± 0.254)
1
2
3
4
0.018 ± 0.003
(0.457 ± 0.076)
S Package 20-Lead Plastic SOL
0.496 – 0.512
(12.598 – 13.005)
17
16
15
14
13
12
11
SEE
0.004 – 0.012 NOTE
(0.102 – 0.305)
LT/GP 0393 10K REV 0
©
LINEAR TECHNOLOGY CORPORATION 1993
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