LT1076IR#PBF [Linear]
LT1076 - Step-Down Switching Regulator; Package: DD PAK; Pins: 7; Temperature Range: -40°C to 85°C;
| 型号： | LT1076IR#PBF |
| 厂家： | 
Linear |
| 描述： | LT1076 - Step-Down Switching Regulator; Package: DD PAK; Pins: 7; Temperature Range: -40°C to 85°C 开关 |
| 文件： | 总16页 (文件大小：182K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1074/LT1076
Step-Down Switching
Regulator
ponents,areincludedonthechip.Thetopologyisaclassic
positive “buck” configuration but several design innova-
tions allow this device to be used as a positive-to-negative
converter, a negative boost converter, and as a flyback
converter. The switch output is specified to swing 40V
below ground, allowing the LT1074 to drive a tapped-
inductor in the buck mode with output currents up to 10A.
FEATURES
■
5A Onboard Switch (LT1074)
■
Operates Up to 60V Input
■
100kHz Switching Frequency
■
Greatly Improved Dynamic Behavior
■
Available in Low Cost 5 and 7-Lead Packages
■
Only 8.5mA Quiescent Current
The LT1074 uses a true analog multiplier in the feedback
loop. This makes the device respond nearly instanta-
neously to input voltage fluctuations and makes loop gain
independent of input voltage. As a result, dynamic behav-
ior of the regulator is significantly improved over previous
designs.
■
Programmable Current Limit
■
Micropower Shutdown Mode
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APPLICATIO S
■
Buck Converter with Output Voltage Range of 2.5V
On-chip pulse by pulse current limiting makes the LT1074
nearlybust-proofforoutputoverloadsorshorts.Theinput
voltage range as a buck converter is 8V to 60V, but a self-
boot feature allows input voltages as low as 5V in the
inverting and boost configurations.
to 50V
■
Tapped-Inductor Buck Converter with 10A Output
at 5V
■
Positive-to-Negative Converter
■
Negative Boost Converter
■
Multiple Output Buck Converter
TheLT1074isavailableinlowcostTO-220orDDpackages
with frequency pre-set at 100kHz and current limit at 6.5A
(LT1076 = 2.6A). A 7-pin TO-220 package is also available
which allows current limit to be adjusted down to zero. In
addition, full micropower shutdown can be programmed.
See Application Note 44 for design details.
U
DESCRIPTIO
The LT®1074 is a 5A (LT1076 is rated at 2A) monolithic
bipolar switching regulator which requires only a few
external parts for normal operation. The power switch, all
oscillator and control circuitry, and all current limit com-
A fixed 5V output, 2A version is also available. See LT1076-5.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Buck Converter Efficiency
TYPICAL APPLICATIO
LT1074
Basic Positive Buck Converter
100
L1**
50µH (LT1074)
V
= 12V, V = 20V
IN
OUT
100µH (LT1076)
90
80
5V
V
V
IN
SW
FB
5A
* USE MBR340 FOR LT1076
** COILTRONICS #50-2-52 (LT1074)
#100-1-52 (LT1076)
PULSE ENGINEERING, INC.
#PE-92114 (LT1074)
10V TO 40V
R1
2.8k
1%
LT1074
MBR745*
V
= 5V, V = 15V
IN
OUT
70
60
50
GND
V
C
#PE-92102 (LT1076)
HURRICANE #HL-AK147QQ (LT1074)
R2
2.21k
1%
R3
L = 50µH TYPE 52 CORE
DIODE = MBR735
2.7k
#HL-AG210LL (LT1076)
+
+
C3†
200µF
C2
0.01µF
C1
500µF
25V
†
RIPPLE CURRENT RATING ≥ I /2
OUT
0
1
2
3
4
5
6
OUTPUT LOAD CURRENT (A)
LT1074•TPC27
LT1074•TA01
sn1074 1074fds
1
LT1074/LT1076
W W
U W
ABSOLUTE AXI U RATI GS
(Note 1)
Input Voltage
I
Pin Voltage (Forced) ............................................ 5.5V
LIM
LT1074/ LT1076 .................................................. 45V
LT1074HV/LT1076HV ......................................... 64V
Switch Voltage with Respect to Input Voltage
Maximum Operating Ambient Temperature Range
Commercial .................................................0°C to 70°C
Industrial ................................................ –40°C to 85°C
Military (OBSOLETE) ..................... –55°C to 125°C
Maximum Operating Junction Temperature Range
Commercial ...............................................0°C to 125°C
Industrial .............................................. –40°C to 125°C
Military (OBSOLETE) .................... –55°C to 150°C
Maximum Storage Temperature ............... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)......................300°C
LT1074/ LT1076 .................................................. 64V
LT1074HV/LT1076HV ......................................... 75V
Switch Voltage with Respect to Ground Pin (V Negative)
LT1074/LT1076 (Note 7) .........................S..W.......... 35V
LT1074HV/LT1076HV (Note 7) ........................... 45V
Feedback Pin Voltage..................................... –2V, +10V
Shutdown Pin Voltage (Not to Exceed VIN) .............. 40V
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W
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PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
ORDER PART
FRONT VIEW
BOTTOM VIEW
5
4
3
2
1
V
V
V
V
V
NUMBER
IN
C
IN
SW
TAB IS
GND
1
4
GND
2
LT1076CQ
LT1076IQ
LT1074CK
CASE
V
C
IS GND
3
LT1074HVCK
LT1074MK
FB/SENSE
Q PACKAGE
5-LEAD PLASTIC DD
FB
SW
LT1074HVMK
LT1076CK
K PACKAGE
4-LEAD TO-3 METAL CAN
LT1076: θJC = 4°C, θJA = 30°C/W
LT1076HVCK
LT1076MK
LT1074: θJC = 2.5°C, θJA = 35°C/W
LT1076: θJC = 4°C, θJA = 35°C/W
LT1076CR
FRONT VIEW
LT1076HVMK
OBSOLETE PACKAGE
7
6
5
4
3
2
1
SHDN
V
FB/SENSE
GND
LT1076IR
Consider the T5 Package for Alternate Source
C
LT1076HVCR
LT1076HVIR
TAB IS
GND
I
V
V
LT1074CT
FRONT VIEW
LIM
SW
IN
LT1074HVCT
LT1074IT
5
4
3
2
1
V
V
IN
SW
R PACKAGE
7-LEAD PLASTIC DD
TAB IS
GND
GND
LT1074HVIT
LT1076CT
V
C
LT1076: θJC = 4°C, θJA = 30°C/W
FB
LT1076HVCT
LT1076IT
T PACKAGE
5-LEAD PLASTIC TO-220
LEADS ARE FORMED STANDARD FOR
STRAIGHT LEADS, ORDER FLOW 06
LT1074CT7
FRONT VIEW
LT1074HVCT7
LT1074IT7
LT1076HVIT
SHDN
7
6
5
4
3
2
1
V
C
FB
GND
LT1074: θJC = 2.5°C, θJA = 50°C/W
LT1076: θJC = 4°C, θJA = 50°C/W
TAB IS
GND
LT1074HVIT7
LT1076CT7
I
LIM
SW
IN
V
V
LT1076HVCT7
T7 PACKAGE
7-LEAD PLASTIC TO-220
LT1074: θJC = 2.5°C, θJA = 50°C/W
LT1076: θJC = 4°C, θJA = 50°C/W
2
*Assumes package is soldered to 0.5 IN of 1 oz. copper over internal ground plane or over back side plane.
Consult LTC Marketing for parts specified with wider operating temperature ranges.
sn1074 1074fds
2
LT1074/LT1076
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. Tj = 25°C, VIN = 25V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch “On” Voltage (Note 2)
LT1074
I
I
I
I
= 1A, T ≥ 0°C
1.85
2.1
2.3
2.5
V
V
V
V
SW
SW
SW
SW
j
= 1A, T < 0°C
j
= 5A, T ≥ 0°C
j
= 5A, T < 0°C
j
LT1076
LT1074
LT1076
I
I
= 0.5A
= 2A
●
●
1.2
1.7
V
V
SW
SW
Switch “Off” Leakage
V
V
≤ 25V, V = 0
5
10
300
500
µA
µA
IN
IN
SW
V
= V
= 0 (Note 8)
MAX, SW
V
V
= 25V, V = 0
150
250
µA
µA
IN
IN
SW
V
= V
= 0 (Note 8)
MAX, SW
Supply Current (Note 3)
V
= 2.5V, V ≤ 40V
●
●
●
8.5
9
140
11
12
300
mA
mA
µA
FB
IN
40V < V < 60V
V
IN
= 0.1V (Device Shutdown) (Note 9)
SHUT
Minimum Supply Voltage
Normal Mode
Startup Mode (Note 4)
●
●
7.3
3.5
8
4.8
V
V
Switch Current Limit (Note 5)
LT1074
I
R
R
Open
●
●
●
5.5
2
6.5
4.5
3
8.5
A
A
A
LIM
= 10k (Note 6)
= 7k (Note 6)
LIM
LIM
LT1076
I
R
R
Open
2.6
1.8
1.2
3.2
A
A
A
LIM
= 10k (Note 6)
= 7k (Note 6)
LIM
LIM
Maximum Duty Cycle
Switching Frequency
85
90
%
90
85
85
100
110
120
125
kHz
kHz
kHz
kHz
T ≤ 125°C
●
●
j
T > 125°C
j
V
= 0V through 2kΩ(Note 5)
20
FB
Switching Frequency Line Regulation
Error Amplifier Voltage Gain (Note 7)
Error Amplifier Transconductance
Error Amplifier Source and Sink Current
8V ≤ V ≤ V
(Note 8)
●
0.03
2000
5000
0.1
%/V
V/V
IN
MAX
1V ≤ V ≤ 4V
C
3700
8000
µmho
Source (V = 2V)
100
0.7
140
1
225
1.6
µA
mA
FB
Sink (V = 2.5V)
FB
Feedback Pin Bias Current
Reference Voltage
V
= V
●
●
0.5
2
µA
V
FB
REF
V = 2V
2.155
2.21
2.265
C
Reference Voltage Tolerance
V
(Nominal) = 2.21V
±0.5
±1
±1.5
±2.5
%
%
REF
All Conditions of Input Voltage, Output
Voltage, Temperature and Load Current
●
●
●
Reference Voltage Line Regulation
8V ≤ V ≤ V
(Note 8)
0.005
0.02
%/V
IN
MAX
V Voltage at 0% Duty Cycle
C
1.5
–4
V
Over Temperature
mV/°C
Multiplier Reference Voltage
Shutdown Pin Current
24
10
V
V
V
= 5V
●
●
5
20
50
µA
µA
SH
SH
≤ V
(≅2.5V)
THRESHOLD
Shutdown Thresholds
Switch Duty Cycle = 0
Fully Shut Down
●
●
2.2
0.1
2.45
0.3
2.7
0.6
V
V
Thermal Resistance Junction to Case
LT1074
LT1076
2.5
4.0
°C/W
°C/W
sn1074 1074fds
3
LT1074/LT1076
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
Note 5: Switch frequency is internally scaled down when the feedback pin
of a device may be impaired.
Note 2: To calculate maximum switch “on” voltage at currents between
voltage is less than 1.3V to avoid extremely short switch on times. During
testing, V is adjusted to give a minimum switch on time of 1µs.
FB
low and high conditions, a linear interpolation may be used.
R
– 1k
R
– 1k
5.5k
LIM
LIM
Note 6: I
≈
(LT1074), I
≈
LIM
(LT1076).
LIM
Note 3: A feedback pin voltage (V ) of 2.5V forces the V pin to its low
2k
Note 7: Switch to input voltage limitation must also be observed.
Note 8: V = 40V for the LT1074/76 and 60V for the LT1074HV/76HV.
FB
C
clamp level and the switch duty cycle to zero. This approximates the zero
load condition where duty cycle approaches zero.
MAX
Note 4: Total voltage from V pin to ground pin must be ≥ 8V after start-
IN
Note 9: Does not include switch leakage.
up for proper regulation.
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BLOCK DIAGRA
INPUT SUPPLY
LT1074
320µA
10µ A
0.3V
+
6V
500Ω
µ-POWER
SHUTDOWN
–
6V TO ALL
CIRCUITRY
REGULATOR
AND BIAS
CURRENT
LIMIT
COMP
0.04
+
–
CURRENT
LIMIT
SHUTDOWN
2.35V
C2
+
–
250Ω
I
*
LIM
SHUTDOWN*
4.5V
10k
FREQ SHIFT
R
R/S
LATCH
100kHz
OSCILLATOR
G1
S
Q
SYNC
R
3V(p-p)
V
IN
+
–
400 Ω
15Ω
Z
C1
+
ANALOG
A1
ERROR
AMP
MULTIPLIER
X
PULSE WIDTH
COMPARATOR
XY
Z
Y
2.21V
–
SWITCH
OUTPUT
(V
)
SW
FB
V
24V (EQUIVALENT)
C
LT1076
0.1Ω
*AVAILABLE ON PACKAGES WITH PIN
COUNTS GREATER THAN 5.
100Ω
SWITCH
OUTPUT (V
)
SW
LT1074 • BD01
sn1074 1074fds
4
LT1074/LT1076
W
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BLOCK DIAGRA DESCRIPTIO
A switch cycle in the LT1074 is initiated by the oscillator
setting the R/S latch. The pulse that sets the latch also
locks out the switch via gate G1. The effective width of this
pulse is approximately 700ns, which sets the maximum
switchdutycycletoapproximately93%at100kHzswitch-
ing frequency. The switch is turned off by comparator C1,
whichresetsthelatch.C1hasasawtoothwaveformasone
input and the output of an analog multiplier as the other
input. The multiplier output is the product of an internal
referencevoltage,andtheoutputoftheerroramplifier,A1,
divided by the regulator input voltage. In standard buck
regulators, this means that the output voltage of A1
required to keep a constant regulated output is indepen-
dent of regulator input voltage. This greatly improves line
transient response, and makes loop gain independent of
input voltage. The error amplifier is a transconductance
type with a GM at null of approximately 5000µmho. Slew
current going positive is 140µA, while negative slew
current is about 1.1mA. This asymmetry helps prevent
overshoot on start-up. Overall loop frequency compensa-
tion is accomplished with a series RC network from VC to
ground.
voltages by feeding the FB signal into the oscillator and
creating a linear frequency downshift when the FB signal
dropsbelow1.3V. Currenttriplevelissetbythevoltageon
the ILIM pin which is driven by an internal 320µA current
source. When this pin is left open, it self-clamps at about
4.5Vandsetscurrentlimitat6.5AfortheLT1074and2.6A
for the LT1076. In the 7-pin package an external resistor
canbeconnectedfromtheILIM pintogroundtosetalower
current limit. A capacitor in parallel with this resistor will
soft-start the current limit. A slight offset in C2 guarantees
thatwhentheILIM pinispulledtowithin200mVofground,
C2outputwillstayhighandforceswitchdutycycletozero.
The “Shutdown” pin is used to force switch duty cycle to
zerobypullingtheILIM pinlow,ortocompletelyshutdown
the regulator. Threshold for the former is approximately
2.35V, and for complete shutdown, approximately 0.3V.
Total supply current in shutdown is about 150µA. A 10µA
pull-up current forces the shutdown pin high when left
open. A capacitor can be used to generate delayed start-
up. A resistor divider will program “undervoltage lockout”
if the divider voltage is set at 2.35V when the input is at the
desired trip point.
Switch current is continuously monitored by C2, which
resets the R/S latch to turn the switch off if an overcurrent
condition occurs. The time required for detection and
switch turn off is approximately 600ns. So minimum
switch “on” time in current limit is 600ns. Under dead
shorted output conditions, switch duty cycle may have to
beaslowas2%tomaintaincontrolofoutputcurrent. This
would require switch on time of 200ns at 100kHz switch-
ing frequency, so frequency is reduced at very low output
The switch used in the LT1074 is a Darlington NPN (single
NPN for LT1076) driven by a saturated PNP. Special
patented circuitry is used to drive the PNP on and off very
quickly even from the saturation state. This particular
switch arrangement has no “isolation tubs” connected to
theswitchoutput, whichcanthereforeswingto40Vbelow
ground.
sn1074 1074fds
5
LT1074/LT1076
U W
TYPICAL PERFOR A CE CHARACTERISTICS
VC Pin Characteristics
VC Pin Characteristics
Feedback Pin Characteristics
2.0
1.5
500
400
200
150
100
50
300
V
ADJUSTED FOR
= 0 AT V = 2V
C
1.0
FB
C
V
FB
≥ 2.5V
200
START OF
FREQUENCY SHIFTING
I
0.5
100
0
0
0
–100
–200
–300
–400
–500
–0.5
–1.0
–1.5
–2.0
–50
–100
–150
–200
SLOPE ≈ 400kΩ
V
≤ 2V
FB
3
0
1
2
3
4
5
6
7
8
9
0
1
2
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
10
VOLTAGE (V)
VOLTAGE (V)
VOLTAGE (V)
LT1074•TPC02
LT1074•TPC01
LT1074•TPC03
Shutdown Pin Characteristics
Shutdown Pin Characteristics
ILIM Pin Characteristics
40
30
0
–5
100
50
T = 25°C
CURRENT FLOWS OUT
OF SHUTDOWN PIN
j
0
20
–10
–15
–20
–25
–30
–35
–40
T
= 25°C
j
–50
V
= 50V
IN
10
–100
–150
–200
–250
–300
–350
–400
SHUTDOWN
THRESHOLD
THIS POINT MOVES
0
WITH V
IN
–10
–20
–30
–40
DETAILS OF THIS
AREA SHOWN IN
OTHER GRAPH
0
10 20 30 40 50 60 70 80
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
–2 –1
0
1
2
3
4
5
6
7
8
VOLTAGE (V)
VOLTAGE (V)
VOLTAGE (V)
LT1074•TPC04
LT1074•PC05
LT1074•TPC06
Supply Current
20
18
16
14
12
10
8
DEVICE NOT SWITCHING
V
= 1V
C
6
4
2
0
0
10
20
30
40
50
60
INPUT VOLTAGE (V)
LT1074•TPC11
sn1074 1074fds
6
LT1074/LT1076
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage vs
Temperature
Supply Current (Shutdown)
Switch “On” Voltage
300
250
2.25
2.24
3.0
2.5
2.0
1.5
1.0
0.5
T = 25°C
j
2.23
2.22
200
150
LT1074
2.21
2.20
2.19
100
50
0
LT1076
2.18
2.17
0
10
20
30
40
50
60
–25
0
25 50
–50
75 100 125 150
0
1
2
3
4
5
6
INPUT VOLTAGE (V)
JUNCTION TEMPERATURE (°C)
SWITCH CURRENT (A)
LT1074•TPC13
LT1074•TPC14
LT1074•TPC28
Switching Frequency vs
Temperature
Reference Shift with Ripple
Voltage
Error Amplifier Phase and GM
20
10
8k
7k
6k
5k
4k
3k
2k
1k
0
200
150
100
50
120
115
110
105
100
95
0
–10
–20
–30
–40
θ
TRI WAVE
SQUARE
WAVE
0
G
M
–50
–100
–150
–200
–50
–60
–70
–80
90
85
80
0
20 40 60 80 100 120 140 160 180 200
1k
10k
100k
1M
10M
–50 –25
0
25 50 75 100 125 150
PEAK-TO-PEAK RIPPLE AT FB PIN (mV)
FREQUENCY (Hz)
JUNCTION TEMPERATURE (°C)
LT1074•TPC18
LT1074•TPC16
LT1074•TPC17
Feedback Pin Frequency Shift
Current Limit vs Temperature*
160
8
7
6
5
4
3
2
1
0
140
120
100
I
PIN OPEN
LIM
R
= 10kΩ
LIM
80
150°C
60
–55°C
R
= 5kΩ
LIM
40
25°C
20
0
*MULTIPLY CURRENTS BY 0.4 FOR LT1076
–50 –25 25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
0
0.5
1.0
1.5
2.0
2.5
3.0
0
FEEDBACK PIN VOLTAGE (V)
LT1074•TPC19
LT1074•TPC22
sn1074 1074fds
7
LT1074/LT1076
U
U
PI DESCRIPTIO S
VIN PIN
∆VGND VOUT
(
)(
)
The VIN pin is both the supply voltage for internal control
circuitry and one end of the high current switch. It is
important, especially at low input voltages, that this pin be
bypassed with a low ESR, and low inductance capacitor to
prevent transient steps or spikes from causing erratic
operation. At full switch current of 5A, the switching
transients at the regulator input can get very large as
shown in Figure 1. Place the input capacitor very close to
the regulator and connect it with wide traces to avoid extra
inductance. Use radial lead capacitors.
∆VOUT
=
2.21
To ensure good load regulation, the ground pin must be
connected directly to the proper output node, so that no
high currents flow in this path. The output divider resistor
should also be connected to this low current connection
line as shown in Figure 2.
LT1074
FB
GND
dl
L
(
)
P
(
(
)
R2
dt
STEP =
I
ESR
) (
)
SW
RAMP =
HIGH CURRENT
RETURN PATH
NEGATIVE OUTPUT NODE
WHERE LOAD REGULATION
WILL BE MEASURED
I
T
ON
(
) (
)
SW
C
LT1074•PD01
LT1074•PD02
Figure 1. Input Capacitor Ripple
Figure 2. Proper Ground Pin Connection
LP = Total inductance in input bypass connections
and capacitor.
FEEDBACK PIN
The feedback pin is the inverting input of an error amplifier
which controls the regulator output by adjusting duty
cycle. The noninverting input is internally connected to a
trimmed 2.21V reference. Input bias current is typically
0.5µA when the error amplifier is balanced (IOUT = 0). The
error amplifier has asymmetrical GM for large input sig-
nals to reduce startup overshoot. This makes the amplifier
more sensitive to large ripple voltages at the feedback pin.
100mVp-p ripple at the feedback pin will create a 14mV
offset in the amplifier, equivalent to a 0.7% output voltage
shift. Toavoidoutputerrors, outputripple(P-P)shouldbe
less than 4% of DC output voltage at the point where the
output divider is connected.
“Spike” height (dI/dt • LP) is approximately 2V per
inch of lead length for LT1074 and 0.8V per inch for
LT1076.
“Step” for ESR = 0.05Ω and ISW = 5A is 0.25V.
“Ramp” for C = 200µF, TON = 5µs, and ISW = 5A,
is 0.12V.
Input current on the VIN Pin in shutdown mode is the sum
of actual supply current (≈140µA, with a maximum of
300µA), and switch leakage current. Consult factory for
special testing if shutdown mode input current is critical.
GROUND PIN
See the “Error Amplifier” section for more details.
It might seem unusual to describe a ground pin, but in the
case of regulators, the ground pin must be connected
properly to ensure good load regulation. The internal
reference voltage is referenced to the ground pin; so any
error in ground pin voltage will be multiplied at the output;
Frequency Shifting at the Feedback Pin
The error amplifier feedback pin (FB) is used to downshift
the oscillator frequency when the regulator output voltage
is low. This is done to guarantee that output short-circuit
sn1074 1074fds
8
LT1074/LT1076
U
U
PI DESCRIPTIO S
current is well controlled even when switch duty cycle
must be extremely low. Theoretical switch “on” time for a
buck converter in continuous mode is:
SHUTDOWN PIN
Theshutdownpinisusedforundervoltagelockout,micro-
power shutdown, soft-start, delayed start, or as a general
purpose on/off control of the regulator output. It controls
switching action by pulling the ILIM pin low, which forces
the switch to a continuous “off” state. Full micropower
shutdown is initiated when the shutdown pin drops below
0.3V.
VOUT + VD
V • f
IN
tON
=
VD = Catch diode forward voltage ( ≈ 0.5V)
f = Switching frequency
The V/I characteristics of the shutdown pin are shown in
Figure 4. For voltages between 2.5V and ≈VIN, a current of
10µA flows out of the shutdown pin. This current in-
creases to ≈25µA as the shutdown pin moves through the
2.35Vthreshold.Thecurrentincreasesfurtherto≈30µAat
the 0.3V threshold, then drops to ≈15µA as the shutdown
voltage fall below 0.3V. The 10µA current source is in-
cluded to pull the shutdown pin to its high or default state
when left open. It also provides a convenient pull-up for
delayed start applications with a capacitor on the shut-
down pin.
At f = 100kHz, tON must drop to 0.2µs when VIN = 25V
and the output is shorted (VOUT = 0V). In current limit,
the LT1074 can reduce tON to a minimum value of
≈0.6µs, much too long to control current correctly for
VOUT = 0. To correct this problem, switching frequency
is lowered from 100kHz to 20kHz as the FB pin drops
from 1.3V to 0.5V. This is accomplished by the circuitry
TO
OSCILLATOR
V
OUT
Q1
+2V
2.21V
When activated, the typical collector current of Q1 in
Figure5, is≈2mA. Asoft-startcapacitorontheILIM pinwill
delay regulator shutdown in response to C1, by
≈(5V)(CLIM)/2mA. Soft-start after full micropower shut-
down is ensured by coupling C2 to Q1.
R1
+
R3
3k
ERROR
AMPLIFIER
–
EXTERNAL
DIVIDER
V
C
FB
R2
2.21k
0
LT1074•PD03
T = 25°C
CURRENT FLOWS OUT
OF SHUTDOWN PIN
j
–5
–10
–15
–20
–25
–30
–35
–40
Figure 3. Frequency Shifting
shown in Figure 3.
SHUTDOWN
THRESHOLD
Q1 is off when the output is regulating (VFB = 2.21V). As
the output is pulled down by an overload, VFB will eventu-
ally reach 1.3V, turning on Q1. As the output continues to
drop, Q1 current increases proportionately and lowers the
frequencyoftheoscillator. Frequencyshiftingstartswhen
the output is ≈ 60% of normal value, and is down to its
minimum value of ≅20kHz when the output is ≅ 20% of
normal value. The rate at which frequency is shifted is
determined by both the internal 3k resistor R3 and the
external divider resistors. For this reason, R2 should not
be increased to more than 4kΩ, if the LT1074 will be
subjected to the simultaneous conditions of high input
voltage and output short-circuit.
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
VOLTAGE (V)
LT1074•PC05
Figure 4. Shutdown Pin Characteristics
sn1074 1074fds
9
LT1074/LT1076
U
U
PI DESCRIPTIO S
V
IN
Hysteresis in undervoltage lockout may be accomplished
by connecting a resistor (R3) from the ILIM pin to the
shutdown pin as shown in Figure 7. D1 prevents the
shutdown divider from altering current limit.
300µA
10µ A
SHUTDOWN
PIN
–
+
I
LIM
PIN
C1
C2
V
IN
R1
2.3V
0.3V
SHUT
LT1074
EXTERNAL
LIM
D1*
Q1
R3
6V
C
I
LIM
–
+
R2
OPTIONAL CURRENT
LIMIT RESISTOR
LT1074•PD09
*1N4148
TO TOTAL
REGULATOR
SHUTDOWN
Figure 7. Adding Hysteresis
LT1074•PD07
Figure 5. Shutdown Circuitry
⎛
⎞
R1
R2
Trip Point = VTP = 2.35V 1+
⎜
⎟
⎝
⎠
Undervoltage Lockout
UndervoltagelockoutpointissetbyR1andR2inFigure 6.
To avoid errors due to the 10µA shutdown pin current, R2
is usually set at 5k, and R1 is found from:
If R3 is added, the lower trip point (VIN descending) will be
the same. The upper trip point (VUTP) will be:
⎛
⎞
⎛
⎞
R1 R1
R2 R3
R1
R3
VUTP = VSH
1
+
− 0.8V
⎜
⎟
⎜
⎟
VTP − VSH
(
)
⎝
⎠
⎝
⎠
R1= R2
VSH
If R1 and R2 are chosen, R3 is given by:
VTP = Desired undervoltage lockout voltage
VSH − 0.8V R1
(
)( )
VSH = Threshold for lockout on the
shutdown pin = 2.45V
R3 =
⎛
⎞
R1
R2
VUTP − VSH 1+
⎜
⎟
Ifquiescentsupplycurrentiscritical, R2maybeincreased
up to 15kΩ, but the denominator in the formula for R2
should replace VSH with VSH – (10µA)(R2).
⎝
⎠
Example: An undervoltage lockout is required such that
the output will not start until VIN = 20V, but will continue
to operate until VIN drops to 15V. Let R2 = 2.32k.
R1
V
IN
SHUT
LT1074
GND
15V − 2.35V
(
)
R1= 2.34k
= 12.5k
(
)
R2
5k
2.35V
2.35 − 0.8 12.5
(
)(
)
R3 =
= 3.9k
LT1074•PD08
⎛
⎞
12.5
2.32
20 − 2.35 1+
Figure 6. Undervoltage Lockout
⎜
⎟
⎝
⎠
sn1074 1074fds
10
LT1074/LT1076
U
U
PI DESCRIPTIO S
ILIM PIN
from forcing current back into the ILIM pin. To calculate a
value for RFB, first calculate RLIM, the RFB:
The ILIM pin is used to reduce current limit below the
preset value of 6.5A. The equivalent circuit for this pin is
shown in Figure 8.
ISC − 0.44 * RL
(
)(
)
RFB =
RL in kΩ
(
)
0.5* RL − 1kΩ − ISC
TO LIMIT
CIRCUIT
(
)
V
IN
320µ A
*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.
D2
Q1
Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k
D1
4.3V
R1
8K
1.5 − 0.44 9kΩ
(
)(
)
D3
6V
RFB =
3.8kΩ
(
)
0.5 9k − 1k − 1.5
(
)
I
LIM
LT1047•PD12
V
OUT
Figure 8. ILIM Pin Circuit
LT1074
FB
I
LIM
When ILIM is left open, the voltage at Q1 base clamps at 5V
through D2. Internal current limit is determined by the
current through Q1. If an external resistor is connected
between ILIM and ground, the voltage at Q1 base can be
reduced for lower current limit. The resistor will have a
voltage across it equal to (320µA)(R), limited to ≈5V when
clamped by D2. Resistance required for a given current
limit is:
R
D2
1N4148
FB
R
LIM
LT1074•PD13
Figure 9. Foldback Current Limit
Error Amplifier
RLIM = ILIM(2kΩ) + 1kΩ (LT1074)
RLIM = ILIM(5.5kΩ) + 1kΩ (LT1076)
The error amplifier in Figure 10 is a single stage design
with added inverters to allow the output to swing above
and below the common mode input voltage. One side of
theamplifieristiedtoatrimmedinternalreferencevoltage
of 2.21V. The other input is brought out as the FB (feed-
back) pin. This amplifier has a GM (voltage “in” to current
“out”) transfer function of ≈5000µmho. Voltage gain is
determined by multiplying GM times the total equivalent
output loading, consisting of the output resistance of Q4
and Q6 in parallel with the series RC external frequency
compensation network. At DC, the external RC is ignored,
and with a parallel output impedance for Q4 and Q6 of
400kΩ, voltage gain is ≈2000. At frequencies above a few
hertz, voltage gain is determined by the external compen-
sation, RC and CC.
As an example, a 3A current limit would require
3A(2k) + 1k = 7kΩ for the LT1074. The accuracy of these
formulas is ±25% for 2A ≤ ILIM ≤ 5A (LT1074) and
7A ≤ ILIM ≤ 1.8A (LT1076), so ILIM should be set at least
25% above the peak switch current required.
Foldback current limiting can be easily implemented by
adding a resistor from the output to the ILIM pin as shown
in Figure 9. This allows full desired current limit (with or
without RLIM) when the output is regulating, but reduces
currentlimitundershort-circuitconditions.Atypicalvalue
for RFB is 5kΩ, but this may be adjusted up or down to set
the amount of foldback. D2 prevents the output voltage
sn1074 1074fds
11
LT1074/LT1076
U
U
PI DESCRIPTIO S
5.8V
Q4
90µ A
µ
90
A
Q3
50µ A
V
D1
C
EXTERNAL
FREQUENCY
COMPENSATION
FB
Q2
Q1
50µA
90µ A
X1.8
D2
R
C
Q6
2.21V
140µ A
C
C
300Ω
LT1074 • PD11
ALL CURRENTS SHOWN ARE AT NULL CONDITION
Figure 10. Error Amplifier
The error amplifier has asymmetrical peak output current.
Q3 and Q4 current mirrors are unity-gain, but the Q6
mirror has a gain of 1.8 at output null and a gain of 8 when
the FB pin is high (Q1 current = 0). This results in a
maximum positive output current of 140µA and a maxi-
mumnegative(sink)outputcurrentof≅1.1mA.Theasym-
metry is deliberate—it results in much less regulator
output overshoot during rapid start-up or following the
release of an output overload. Amplifier offset is kept low
by area scaling Q1 and Q2 at 1.8:1.
Gm
2π • f • CC
AV = Gm • RC at high frequencies
AV =
at mid frequencies
Phase shift from the FB pin to the VC pin is 90° at mid
frequencies where the external CC is controlling gain, then
drops back to 0° (actually 180° since FB is an inverting
input) when the reactance of CC is small compared to RC.
The low frequency “pole” where the reactance of CC is
equal to the output impedance of Q4 and Q6 (rO), is:
Amplifier swing is limited by the internal 5.8V supply for
positive outputs and by D1 and D2 when the output goes
low. Low clamp voltage is approximately one diode drop
(≈0.7V – 2mV/°C).
1
fPOLE
=
r ≈ 400kΩ
O
2π • r • C
O
Although fPOLE varies as much as 3:1 due to rO variations,
mid-frequency gain is dependent only on Gm, which is
specified much tighter on the data sheet. The higher
frequency “zero” is determined solely by RC and CC.
Note that both the FB pin and the VC pin have other internal
connections. Refer to the frequency shifting and synchro-
nizing discussions.
1
fZERO
=
2π • RC • CC
sn1074 1074fds
12
LT1074/LT1076
U
TYPICAL APPLICATIO S
Tapped-Inductor Buck Converter
L2
5µH
L1*
V
V
OUT
IN
V
V
SW
20V† TO 35V
5V, 10A†
IN
3
1
D2
35V
5W
R1
D1**
LT1074HV
2.8k
C1
4400
(2 EA
+
µ
F
FB
GND
V
C
2200µF,
+
C4
390
16V
D3
R2
2.21k
R3
1k
C2
0.2
16V)
µ
F
1N5819
+
C3
0.01µF
µF
200
µF
50V
* PULSE ENGINEERING #PE±65282
** MOTOROLA MBR2030CTL
†
IF INPUT VOLTAGE IS BELOW 20V,
MAXIMUM OUTPUT CURRENT WILL BE REDUCED. SEE AN44
LT1074 •TA02
Positive-to-Negative Converter with 5V Output
V
IN
+
C1
4.5V to
40V
220
µ
F
50V
+
L1
25
µ
H
5A††
R3*
2.74k
R1**
5.1k
V
IN
V
SW
+
C2
LT1074
1000
µF
R2**
10k
10V
OPTIONAL FILTER
V
FB
V
C
GND
D1†
MBR745
5µH
–
+
200
µF
R4
1.82k*
10V
C3
0.1
C4**
0.01
µ
F
µ
F
–5V,1A***
†
* = 1% FILM RESISTORS
D1 = MOTOROLA-MBR745
LOWER REVERSE VOLTAGE RATING MAY BE USED FOR LOWER INPUT VOLTAGES.
LOWER CURRENT RATING IS ALLOWED FOR LOWER OUTPUT CURRENT. SEE AN44.
C1 = NICHICON-UPL1C221MRH6
C2 = NICHICON-UPL1A102MRH6
L1 = COILTRONICS-CTX25-5-52
††
LOWER CURRENT RATING MAY BE USED FOR LOWER OUTPUT CURRENT. SEE AN44.
** R1, R2, AND C4 ARE USED FOR LOOP FREQUENCY COMPENSATION WITH LOW INPUT VOLTAGE,
BUT R1 AND R2 MUST BE INCLUDED IN THE CALCULATION FOR OUTPUT VOLTAGE DIVIDER VALUES.
FOR HIGHER OUTPUT VOLTAGES, INCREASE R1, R2, AND R3 PROPORTIONATELY.
FOR INPUT VOLTAGE > 10V, R1, R2, AND C4 CAN BE ELIMINATED, AND COMPENSATION IS
DONE TOTALLY ON THE V PIN.
C
R3 =
R1 = (R3) (1.86)
–2.37 (KΩ)
V
OUT
R2 = (R3) (3.65)
** MAXIMUM OUTPUT CURRENT OF 1A IS DETERMINED BY MINIMUM INPUT
VOLTAGE OF 4.5V. HIGHER MINIMUM INPUT VOLTAGE WILL ALLOW MUCH HIGHER
OUTPUT CURRENTS. SEE AN44.
LT1074 • TA03
sn1074 1074fds
13
LT1074/LT1076
U
PACKAGE DESCRIPTIO
K Package
4-Lead TO-3 Metal Can
(Reference LTC DWG # 05-08-1311)
0.760 – 0.775
(19.30 – 19.69)
0.320 – 0.350
(8.13 – 8.89)
0.060 – 0.135
(1.524 – 3.429)
0.420 – 0.480
(10.67 – 12.19)
0.038 – 0.043
(0.965 – 1.09)
1.177 – 1.197
(29.90 – 30.40)
0.655 – 0.675
(16.64 – 19.05)
0.470 TP
P.C.D.
0.151 – 0.161
(3.84 – 4.09)
DIA 2 PLC
0.167 – 0.177
(4.24 – 4.49)
R
0.490 – 0.510
(12.45 – 12.95)
R
72°
18°
K4(TO-3) 1098
OBSOLETE PACKAGE
Q Package
5-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1461)
0.060
(1.524)
TYP
0.390 – 0.415
(9.906 – 10.541)
0.060
(1.524)
0.165 – 0.180
(4.191 – 4.572)
0.256
(6.502)
0.045 – 0.055
(1.143 – 1.397)
15° TYP
+0.008
0.004
–0.004
0.060
(1.524)
0.183
(4.648)
0.059
(1.499)
TYP
0.330 – 0.370
(8.382 – 9.398)
+0.203
–0.102
0.102
(
)
0.095 – 0.115
(2.413 – 2.921)
0.075
(1.905)
0.067
(1.70)
BSC
0.050 ± 0.012
(1.270 ± 0.305)
0.300
(7.620)
0.013 – 0.023
(0.330 – 0.584)
+0.012
0.143
–0.020
0.028 – 0.038
(0.711 – 0.965)
+0.305
BOTTOM VIEW OF DD PAK
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
3.632
Q(DD5) 1098
(
)
–0.508
sn1074 1074fds
14
LT1074/LT1076
U
PACKAGE DESCRIPTIO
R Package
7-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1462)
0.060
(1.524)
TYP
0.390 – 0.415
(9.906 – 10.541)
0.060
0.256
0.165 – 0.180
(4.191 – 4.572)
(1.524)
(6.502)
0.045 – 0.055
(1.143 – 1.397)
15° TYP
+0.008
0.004
–0.004
0.060
0.183
0.059
(1.499)
TYP
0.330 – 0.370
(8.382 – 9.398)
(1.524)
(4.648)
+0.203
–0.102
0.102
(
)
0.095 – 0.115
(2.413 – 2.921)
0.075
(1.905)
0.050
(1.27)
BSC
0.050 ± 0.012
(1.270 ± 0.305)
0.300
(7.620)
0.013 – 0.023
(0.330 – 0.584)
+0.012
0.143
–0.020
0.026 – 0.036
(0.660 – 0.914)
+0.305
BOTTOM VIEW OF DD PAK
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
3.632
(
)
–0.508
R (DD7) 1098
T Package
5-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1421)
0.165 – 0.180
(4.191 – 4.572)
0.147 – 0.155
(3.734 – 3.937)
DIA
0.390 – 0.415
(9.906 – 10.541)
0.045 – 0.055
(1.143 – 1.397)
0.230 – 0.270
(5.842 – 6.858)
0.570 – 0.620
(14.478 – 15.748)
0.620
(15.75)
TYP
0.460 – 0.500
(11.684 – 12.700)
0.330 – 0.370
(8.382 – 9.398)
0.700 – 0.728
(17.78 – 18.491)
0.095 – 0.115
(2.413 – 2.921)
SEATING PLANE
0.152 – 0.202
(3.861 – 5.131)
0.155 – 0.195*
(3.937 – 4.953)
0.260 – 0.320
(6.60 – 8.13)
0.013 – 0.023
(0.330 – 0.584)
0.067
BSC
0.135 – 0.165
(3.429 – 4.191)
0.028 – 0.038
(0.711 – 0.965)
(1.70)
* MEASURED AT THE SEATING PLANE
T5 (TO-220) 0399
sn1074 1074fds
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-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
15
LT1074/LT1076
U
TYPICAL APPLICATIO
Negative Boost Converter
R1
12.7k
100pF
V
IN
V
FB
R2
2.21k
LT1074
V
SW
V
C
GND
C3
+
+
C1
200µF
1000
µ
F
L1
25
C2
15V
D1*
25V
µ
H
1nF
0.01µF
R3
750Ω
V
OUT
–15V**
V
IN
–5V TO –15V
*
**
MBR735
I
(MAX) = 1A TO 3A DEPENDING
+
OUT
100µF
ON INPUT VOLTAGE. SEE AN44
5µH
OPTIONAL OUTPUT FILTER
LT1074 • TA04
U
PACKAGE DESCRIPTIO
T7 Package
7-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1422)
0.165 – 0.180
(4.191 – 4.572)
0.147 – 0.155
(3.734 – 3.937)
DIA
0.390 – 0.415
(9.906 – 10.541)
0.045 – 0.055
(1.143 – 1.397)
0.230 – 0.270
(5.842 – 6.858)
0.570 – 0.620
(14.478 – 15.748)
0.620
(15.75)
TYP
0.460 – 0.500
(11.684 – 12.700)
0.330 – 0.370
(8.382 – 9.398)
0.700 – 0.728
(17.780 – 18.491)
0.095 – 0.115
(2.413 – 2.921)
0.155 – 0.195*
(3.937 – 4.953)
SEATING PLANE
0.152 – 0.202
(3.860 – 5.130)
0.260 – 0.320
(6.604 – 8.128)
0.013 – 0.023
(0.330 – 0.584)
0.050
BSC
0.026 – 0.036
(0.660 – 0.914)
(1.27)
0.135 – 0.165
(3.429 – 4.191)
*MEASURED AT THE SEATING PLANE
T7 (TO-220) 0399
RELATED PARTS
PART NUMBER
LT1375/LT1376
LT1374/LT1374HV
LT1370
DESCRIPTION
COMMENTS
1.5A, 500kHz Step-Down Switching Regulators
4.5A, 500kHz Step-Down Switching Regulators
6A, 500kHz High Efficiency Switching Regulator
Wide Input Range, High Efficiency Step-Down Regulator
High Power Synchronous DC/DC Controller
3A, 1.25MHz, Step-Down Regulator
V
IN
V
IN
Up to 25V, I
Up to 1.25A, SO-8
OUT
Up to 25V (32V for HV), I
Up to 4.25A, SO-8/DD
OUT
6A/42V Internal Switch, 7-Lead DD/TO-220
LT1676
V
IN
V
IN
V
IN
from 7.4V to 60V, I
Up to 0.5A, SO-8
OUT
LT1339
Up to 60V, I
Up to 50A, Current Mode
OUT
LT1765
= 3V to 25V, V =1.2V, TSSOP-16E, SO8 Package
µF
sn1074 1074fds
LT/CPI 0202 1.5K REV D • PRINTED IN USA
16 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1994
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SI9137
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