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PDF LT3436 Data sheet ( Hoja de datos )
| Número de pieza | LT3436 | |
| Descripción | 3A & 800kHz Step-Up Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT3436www.DataSheet4U.com
3A, 800kHz Step-Up
Switching Regulator
FEATURES
■ Constant 800kHz Switching Frequency
■ Wide Operating Voltage Range: 3V to 25V
■ High Efficiency 0.1Ω/3A Switch
■ 1.2V Feedback Reference Voltage
■ ±2% Overall Output Voltage Tolerance
■ Uses Low Profile Surface Mount External
Components
■ Low Shutdown Current: 11µA
■ Synchronizable from 1MHz to 1.4MHz
■ Current-Mode Control
■ Constant Maximum Switch Current Rating
at All Duty Cycles*
■ Available in a Small Thermally Enhanced
TSSOP-16 Package
U
APPLICATIO S
■ DSL Modems
■ Portable Computers
■ Battery-Powered Systems
■ Distributed Power
DESCRIPTIO
The LT®3436 is an 800kHz monolithic boost switching
regulator. A high efficiency 3A, 0.1Ω switch is included on
the die together with all the control circuitry required to
complete a high frequency, current-mode switching regu-
lator. Current-mode control provides fast transient re-
sponse and excellent loop stability.
New design techniques achieve high efficiency at high
switching frequencies over a wide operating range. A low
dropout internal regulator maintains consistent perfor-
mance over a wide range of inputs from 24V systems to Li-
Ion batteries. An operating supply current of 1mA main-
tains high efficiency, especially at lower output currents.
Shutdown reduces quiescent current to 11µA. Maximum
switch current remains constant at all duty cycles. Syn-
chronization capability allows an external logic level signal
to increase the internal oscillator from 1MHz to 1.4MHz.
Full cycle-by-cycle switch current limit protection and ther-
mal shutdown are provided. High frequency operation al-
lows the reduction of input and output filtering components
and permits the use of tiny chip inductors. The LT3436 is
available in an exposed pad, 16-pin TSSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Protectd by U.S. Patents including 6535042, 6611131, 6498466
TYPICAL APPLICATIO
5V to 12V Boost Converter
3.9µH
INPUT
5V
4.7µF
CERAMIC
OPEN
OR
HIGH
= ON
VIN VSW
LT3436
SHDN
FB
SYNC GND VC
10nF
4.7k
B220A
90.9k
470pF
10k
1%
†MAXIMUM OUTPUT CURRENT IS SUBJECT TO THERMAL DERATING.
OUTPUT
12V
0.9A†
22µF
CERAMIC
3436 TA01
Efficiency vs Load Current
90 VIN = 5V
VOUT = 12V
85
80
75
70
65
60
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
LOAD CURRENT (A)
3436 TA01b
3436fa
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LT3436www.DataSheet4U.com
BLOCK DIAGRAM
The LT3436 is a constant frequency, current-mode boost
converter. This means that there is an internal clock and
two feedback loops that control the duty cycle of the power
switch. In addition to the normal error amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscilla-
tor pulse which sets the RS flip-flop to turn the switch on.
When switch current reaches a level set by the inverting
input of the comparator, the flip-flop is reset and the
switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
and output capacitor, then an abrupt 180° shift will occur.
The current fed system will have 90° phase shift at a much
lower frequency, but will not have the additional 90° shift
until well beyond the LC resonant frequency. This makes
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
A comparator connected to the shutdown pin disables the
internal regulator, reducing supply current.
INPUT
2.5V BIAS
REGULATOR
INTERNAL
VCC
SLOPE COMP
SYNC
800kHz
OSCILLATOR
SHUTDOWN
COMPARATOR
+
–
1.35V
7µA
Σ
0.3V
CURRENT
COMPARATOR
+
–
S
RS
FLIP-FLOP
R
DRIVER
CIRCUITRY
CURRENT SENSE
AMPLIFIER VOLTAGE
GAIN = 40
+
–
Q1
POWER
SWITCH
SW
0.005Ω
SHDN
3µA
ERROR
VC AMPLIFIER
gm = 850µMho
Figure 1. Block Diagram
1.2V
FB
GND
3436 F01
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LT3436www.DataSheet4U.com
APPLICATIONS INFORMATION
The inductor must have a rating greater than its peak
operating current to prevent saturation resulting in effi-
ciency loss. Peak inductor current is given by:
ILPEAK
=
(VOUT )(IOUT )
VIN • η
+
VIN(VOUT − VIN)
2VOUT (L)(f)
Also, consideration should be given to the DC resistance
of the inductor. Inductor resistance contributes directly to
the efficiency losses in the overall converter.
thermal resistance number and add in worst-case ambient
temperature:
TJ = TA + θJA (PTOT)
If a true die temperature is required, a measurement of
the SYNC to GND pin resistance can be used. The SYNC
pin resistance across temperature must first be cali-
brated, with no device power, in an oven. The same
measurement can then be used in operation to indicate the
die temperature.
THERMAL CALCULATIONS
Power dissipation in the LT3436 chip comes from four
sources: switch DC loss, switch AC loss, drive current, and
input quiescent current. The following formulas show how
to calculate each of these losses. These formulas assume
continuous mode operation, so they should not be used
for calculating efficiency at light load currents.
DC, duty cycle = (VOUT − VIN)
VOUT
ISW
=
(VOUT )(IOUT )
VIN
Switch loss:
( )( )PSW = (DC)(ISW )2(RSW ) + 17n ISW VOUT (f)
VIN loss:
PVIN
=
(VIN)(ISW )(DC)
50
+ 1mA(VIN)
RSW = Switch resistance (≈ 0.16Ω hot)
Example: VIN = 5V, VOUT = 12V and IOUT = 0.8A
FREQUENCY COMPENSATION
Loop frequency compensation is performed on the output
of the error amplifier (VC pin) with a series RC network.
The main pole is formed by the series capacitor and the
output impedance (≈500kΩ) of the error amplifier. The
pole falls in the range of 2Hz to 20Hz. The series resistor
creates a “zero” at 1kHz to 5kHz, which improves loop
stability and transient response. A second capacitor, typi-
cally one-tenth the size of the main compensation capaci-
tor, is sometimes used to reduce the switching frequency
ripple on the VC pin. VC pin ripple is caused by output
voltage ripple attenuated by the output divider and multi-
plied by the error amplifier. Without the second capacitor,
VC pin ripple is:
VC
Pin
Ripple
=
1.2(VRIPPLE)(gm)(RC)
(VOUT)
VRIPPLE = Output ripple (VP–P)
gm = Error amplifier transconductance
(≈850µmho)
RC = Series resistor on VC pin
VOUT = DC output voltage
Total power dissipation = 0.34 + 0.31 + 0.11 + 0.005 =
0.77W
Thermal resistance for LT3436 package is influenced by
the presence of internal or backside planes. With a full
plane under the package, thermal resistance will be about
40°C/W. To calculate die temperature, use the appropriate
To prevent irregular switching, VC pin ripple should be
kept below 50mVP–P. Worst-case VC pin ripple occurs at
maximum output load current and will also be increased if
poor quality (high ESR) output capacitors are used. The
addition of a 150pF capacitor on the VC pin reduces
switching frequency ripple to only a few millivolts. A low
value for RC will also reduce VC pin ripple, but loop phase
margin may be inadequate.
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT3436.PDF ] | |
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