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PDF LT1076HVIT Data sheet ( Hoja de datos )
| Número de pieza | LT1076HVIT | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1074/LT1076
Step-Down Switching
Regulator
FEATURES
s 5A Onboard Switch (LT1074)
s 100kHz Switching Frequency
s Greatly Improved Dynamic Behavior
s Available in Low Cost 5 and 7-Lead Packages
s Only 8.5mA Quiescent Current
s Programmable Current Limit
s Operates Up to 60V Input
s Micropower Shutdown Mode
U
APPLICATIO S
s Buck Converter with Output Voltage Range of 2.5V
to 50V
s Tapped-Inductor Buck Converter with 10A Output
at 5V
s Positive-to-Negative Converter
s Negative Boost Converter
s Multiple Output Buck Converter
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-
ponents, are included on the chip. The topology is a classic
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.
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.
On-chip pulse by pulse current limiting makes the LT1074
nearly bust-proof for output overloads or shorts. The input
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.
The LT1074 is available in low cost TO-220 or TO-3
packages 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.
A fixed 5V output, 2A version is also available. See LT1076-5.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Basic Positive Buck Converter
10V TO 40V
+
L1**
50µH (LT1074)
100µH (LT1076)
VIN VSW
LT1074
GND
C3†
200µF
FB
VC
R3
2.7k
C2
0.01µF
MBR745*
R1
2.8k
1%
R2
2.21k
1% +
5V
5A * USE MBR340 FOR LT1076
** COILTRONICS #50-2-52 (LT1074)
#100-1-52 (LT1076)
PULSE ENGINEERING, INC.
#PE-92114 (LT1074)
#PE-92102 (LT1076)
HURRICANE #HL-AK147QQ (LT1074)
#HL-AG210LL (LT1076)
C1 † RIPPLE CURRENT RATING ≥ IOUT/2
500µF
25V
LT1074•TA01
Buck Converter Efficiency
100
90
80
70
60
50
0
LT1074
VOUT = 12V, VIN = 20V
VOUT = 5V, V IN = 15V
L = 50µH TYPE 52 CORE
DIODE = MBR735
1 234 5 6
OUTPUT LOAD CURRENT (A)
LT1074•TPC27
1
1 page  
LT1074/LT1076
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
switch duty cycle to approximately 93% at 100kHz switch-
ing frequency. The switch is turned off by comparator C1,
which resets the latch. C1 has a sawtooth waveform as one
input and the output of an analog multiplier as the other
input. The multiplier output is the product of an internal
reference voltage, and the output of the error amplifier, 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.
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
be as low as 2% to maintain control of output current. This
would require switch on time of 200ns at 100kHz switch-
ing frequency, so frequency is reduced at very low output
voltages by feeding the FB signal into the oscillator and
creating a linear frequency downshift when the FB signal
drops below 1.3V. Current trip level is set by the voltage on
the ILIM pin which is driven by an internal 320µA current
source. When this pin is left open, it self-clamps at about
4.5V and sets current limit at 6.5A for the LT1074 and 2.6A
for the LT1076. In the 7-pin package an external resistor
can be connected from the ILIM pin to ground to set a lower
current limit. A capacitor in parallel with this resistor will
soft-start the current limit. A slight offset in C2 guarantees
that when the ILIM pin is pulled to within 200mV of ground,
C2 output will stay high and force switch duty cycle to zero.
The “Shutdown” pin is used to force switch duty cycle to
zero by pulling the ILIM pin low, or to completely shut down
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.
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
the switch output, which can therefore swing to 40V below
ground.
5
5 Page 
LT1074/LT1076
PI DESCRIPTIO S
ILIM PIN
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.
TO LIMIT
CIRCUIT
VIN
320µ A
D2
Q1
D1
R1
8K D3
6V
4.3V
I LIM
LT1047•PD12
Figure 8. ILIM Pin Circuit
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:
RLIM = ILIM(2kΩ) + 1kΩ (LT1074)
RLIM = ILIM(5.5kΩ) + 1kΩ (LT1076)
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
current limit under short-circuit conditions. A typical value
for RFB is 5kΩ, but this may be adjusted up or down to set
the amount of foldback. D2 prevents the output voltage
from forcing current back into the ILIM pin. To calculate a
value for RFB, first calculate RLIM, the RFB:
RFB
=
(ISC − 0.44*)(RL )
0.5*(RL − 1kΩ) − ISC
(RL
in
kΩ)
*Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076.
Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k
RFB
=
(1.5 − 0.44)(9kΩ)
0.5(9k − 1k) − 1.5
(3.8kΩ)
LT1074
FB
I LIM
VOUT
RLIM
RFB D2
1N4148
LT1074•PD13
Figure 9. Foldback Current Limit
Error Amplifier
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
the amplifier is tied to a trimmed internal reference voltage
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.
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1076HVIT.PDF ] | |
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