LT1676 Linear Technology, LT1676 Datasheet
LT1676
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LT1676 Summary of contents
Page 1
... The internal control circuitry is normally powered via the V CC from the V action of the LT1676 switch circuitry is also load depen- dent. At medium to high loads, the output switch circuitry maintains high rise time for good efficiency. At light loads, rise time is deliberately reduced to avoid pulse skipping behavior ...
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... Pin Voltage ....................................................... 30V CC FB Pin Voltage ........................................................... 3V Operating Junction Temperature Range LT1676C ................................................ 125 C LT1676I ............................................ – 125 C Storage Temperature Range ................. – 150 C Lead Temperature (Soldering, 10 sec).................. 300 C ELECTRICAL CHARACTERISTICS The denotes specifications which apply over the full operating temperature range, otherwise specifications are T ...
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... TEMPERATURE ( C) LT1676 G01 CONDITIONS High dV/dt Mode (Note 4) L Switching Action On Switching Action Off SHDN V = 1.25V SHDN Note 3: Switch current limit is DC trimmed and tested in production. Inductor dl/dt rate will cause a somewhat higher current limit in actual application ...
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... Switch Minimum On-Time vs Temperature 600 V = 48V 500 FB = 400 300 200 100 0 50 100 125 –50 – TEMPERATURE ( C) 1676 G09 Error Amplifier Transconductance vs Temperature 750 700 650 600 550 500 450 400 –50 – 100 125 TEMPERATURE ( C) LT1676 G12 ...
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... Frequency compensation of the over- all loop is effected by placing a capacitor, (or in most cases a series RC combination) between this node and ground SWDR SWON BOOST SWOFF 1676 TD01 LT1676 C pin is left CC Low dV/dt Mode 1676 TD02 5 ...
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... OPERATIO The LT1676 is a current mode switching regulator IC that has been optimized for high efficiency operation in high input voltage, low output voltage Buck topologies. The Block Diagram shows an overall view of the system. Several of the blocks are straightforward and similar to those found in traditional designs, including: Internal Bias Regulator, Oscillator and Feedback Amplifier ...
Page 7
... In a typical application, proper inductance value is dictated by matching the discontinuous/continuous crossover point with the LT1676 internal low-to-high dV/dt threshold. This is the best compromise between maintaining control with light loads while maintaining good efficiency with heavy loads ...
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... OUT User selection of an appropriate output capacitor is rela- tively easy, as this capacitor sees only the AC ripple current in the inductor. As the LT1676 is designed for Buck or step-down applications, output voltage will nearly always be compatible with tantalum type capacitors, which are generally available in ratings up to 35V or so. These tantalum types offer good volumetric efficiency and many are available with specified ESR performance ...
Page 9
... Burst Mode operation behavior (see Typical Applications and Figure 8). Maximum Load/Short-Circuit Considerations The LT1676 is a current mode controller. It uses the V node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the V node, nominally 2V, then acts as an output switch peak current limit ...
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... IN VCC OUT (This assumes that the V pin is connected Power loss internal to the LT1676 related to actual output current is composed of both DC and AC switching losses. These can be roughly estimated as follows: DC switching losses are dominated by output switch “ON voltage”, i.e Output switch ON voltage, typically 1V at 500mA ...
Page 11
... The somewhat arbitrary value of 200 A was chosen to be significantly above the SHDN pin input current to minimize its error contribution, but significantly below the typical 3.2mA the LT1676 draws in lockout mode. Resistor R4 is then chosen to yield this same 200 A, less 2.5 A, with the LT1676 and FB ...
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... SHDN pin input current.) Behavior is as follows: Normal operation is observed at the nominal input voltage of 48V. As the input voltage is decreased to roughly 43V, switching action will stop, V will drop to zero, and the LT1676 will draw its V quiescent currents from the V supply much lower IN ...
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... R4/ increased, thereby minimizing power drain. Hysteresis is externally programmable via resistor divider R6/R7. The LTC1440 output directly controls the LT1676 via its shutdown pin, driving it to either 5V (ON (Full Shutdown). A simple linear voltage regulator to power the LTC1440 is provided by Q1, Q2 and R7. Just below the UVLO threshold, nominally 43V, total current drain is typically 50 A ...
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... U2. Burst Mode Operation Configuration with UVLO Figure 7a uses an external comparator to control the LT1676 via its SHDN pin. As such, the user’s ability to set an undervoltage lockout (UVLO) threshold with a resistor divider from V to SHDN pin to ground is lost. This ability ...
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... F08 0.400* (10.160) MAX 0.255 0.015* (6.477 0.381) 0.020 MIN N8 1197 0.189 – 0.197* (4.801 – 5.004 0.150 – 0.157** 0.228 – 0.244 (3.810 – 3.988) (5.791 – 6.197 LT1676 SO8 0996 15 ...
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... Operation Up to 60V, High Power Anti-Shoot-Through Drivers Operation Up to 25V Input, Synchronizable (LT1375) Transforms Switching Regulators Into High Efficiency Battery Chargers LT1676 with 200kHz Switching Frequency (High Current Applications Generally Restricted to 40V) Operation up to 48V, Controlled Voltage and Current Slew Rates V ...