LT1956 LINER [Linear Technology], LT1956 Datasheet
LT1956
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LT1956 Summary of contents
Page 1
... A shutdown pin reduces supply current and the device can be externally synchronized from 580kHz to 700kHz with a logic level input. The LT1956/LT1956-5 are available in fused-lead 16-pin SSOP and thermally enhanced TSSOP packages. , LTC and LT are registered trademarks of Linear Technology Corporation. *U.S. PATENT NO. 6,498,466 MMSD914TI 0 ...
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... LT1956/LT1956 ABSOLUTE AXI U RATI GS Input Voltage (V ) ................................................. 60V IN BOOST Pin Above SW ............................................ 35V BOOST Pin Voltage ................................................. 68V SYNC, SENSE Voltage (LT1956-5) ........................... 7V SHDN Voltage ........................................................... 6V BIAS Pin Voltage .................................................... 30V FB Pin Voltage/Current (LT1956) ................... 3.5V/2mA U PACKAGE/ORDER I FOR ATIO TOP VIEW 1 GND 16 GND SHDN SYNC ...
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... Specifications over the – 125 C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT1956IFE/LT1956IFE-5/ LT1956IGN/LT1956IGN-5 are guaranteed over the full – 125 C operating junction temperature range. Note 9: Transconductance and voltage gain refer to the internal amplifier exclusive of the voltage divider ...
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... LT1956/LT1956 TYPICAL PERFOR A CE CHARACTERISTICS Switch Peak Current Limit 2.5 TYPICAL 2.0 GUARANTEED MINIMUM 1.5 1 100 DUTY CYCLE (%) 1956 G01 Lockout and Shutdown Thresholds 2.4 LOCKOUT 2.0 1.6 1.2 0.8 START-UP 0.4 SHUTDOWN 0 –50 – 100 125 JUNCTION TEMPERATURE ( C) 1956 G04 Error Amplifier Transconductance 2500 ...
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... T = 125 C J 350 300 250 200 150 T = – 100 0.5 1 1.5 SWITCH CURRENT (A) 1766 G14 LT1956/LT1956-5 BOOST Pin Current 0.5 1 1.5 SWITCH CURRENT (A) 1956 G12 Switch Minimum ON Time vs Temperature 600 500 400 300 ...
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... LT1956/LT1956 CTIO S GND (Pins 16): The GND pin connections act as the reference for the regulated output, so load regulation will suffer if the “ground” end of the load is not at the same voltage as the GND pins of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pins and the load ground ...
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... Most of the circuitry of the LT1956 operates from an internal 2.9V bias line. The bias regulator normally draws power from the regulator input pin, but if the BIAS pin is connected to an external voltage higher than 3V, bias power will be drawn from the external source (typically the regulated output voltage) ...
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... U U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1956 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin ...
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... Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1956 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage. When choosing an inductor you will need to consider output ripple voltage, maximum load current, peak induc- tor current and fault current in the inductor ...
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... Dead shorts will actually be more gentle on the /L IN inductor because the LT1956 has frequency and current limit foldback. Peak inductor and switch current can be significantly higher than output current, especially with smaller induc- tors and lighter loads, so don’ ...
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... MAXIMUM OUTPUT LOAD CURRENT Maximum load current for a buck converter is limited by the maximum switch current rating (I for the LT1956 is 1.5A. Unlike most current mode convert- ers, the LT1956 maximum switch current limit does not fall off at high duty cycles. Most current mode converters suffer a drop off of peak switch current for duty cycles above 50% ...
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... If a greater input voltage is required, increasing the resistance in series with the inductor may suffice (see short-circuit calculations at the end of this section). Alternatively, the 1.5A LT1766 can be used since it is identical to the LT1956 but runs at a lower frequency ) low; of 200kHz, allowing higher sustained input voltage capa- LP-P bility during output short circuit ...
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... This lower oscillator frequency during short-circuit condi- tions can then maintain control with the effective mini- mum on time. Even with frequency foldback, however, the LT1956 will not survive a permanent output short at the absolute maximum voltage rating of V defined solely by internal semiconductor junction break- down effects ...
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... The rise and fall times of these pulses are very fast. The input capacitor is required to reduce the voltage ripple this causes at the input of LT1956 and force the switching current into a tight local loop, thereby minimiz- ing EMI. The RMS ripple current can be calculated from: ...
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... SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1956. Typically, UVLO is used in situations where the input supply is current limited , or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops ...
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... R 116 387 LT1956 + 2.38V IN – 5.5 A SHDN + – 0.4V GND Figure 4. Undervoltage Lockout SYNCHRONIZING The SYNC input must pass from a logic level low, through can be FB the maximum synchronization threshold with a duty cycle between 10% and 90%. The input can be driven directly from a logic level output ...
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... LT1956 switch. When operating at higher currents and input voltages, with poor layout, this spike can generate volt- ages across the LT1956 that may exceed its absolute maximum rating. A ground plane should always be used under the switcher circuitry to prevent interplane coupling and overall noise ...
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... Board layout also has a significant effect on thermal resis- tance. For the GN package, Pins and 16, GND, are a continuous copper plate that runs under the LT1956 die. This is the best thermal path for heat out of the package. Reducing the thermal resistance from Pins and 16 onto the board will reduce die temperature and increase the power capability of the LT1956 ...
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... AC losses, quiescent and catch diode losses, a lower V IN losses. In general, the maximum and minimum V should be checked with maximum typical load current for calculation of the LT1956 die temperature more accurate die temperature is required, a measurement of ) will be about JA the SYNC pin resistance (to GND) can be used. The SYNC pin resistance can be measured by forcing a voltage no greater than 0 ...
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... • / • • / • 0 084 . W 20 exceeds 3. BOOST BOOST LT1956 SW IN SHDN BIAS R1 SYNC FB R2 GND Figure 9. BOOST Pin, Diode Selection . DROOP V OUT + C1 1956 F09 1956f ...
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... The absolute maximum input supply voltage for the LT1956 is specified at 60V. This is based on internal semiconduc- tor junction breakdown effects. The practical maximum input supply voltage for the LT1956 may be less than 60V due to internal power dissipation or switch minimum on time considerations. For the extreme case of an output short-circuit fault to ground, see the section Short-Circuit Considerations ...
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... F pin to ground C In systems with a primary and backup supply, for ex- ample, a battery powered device with a wall adapter input, the output of the LT1956 can be held up by the backup ) at the V supply with the LT1956 input disconnected. In this condi tion, the SW pin will source current into the V SHDN pin is held at ground, only the shut down current will be pulled via the SW pin from the second supply ...
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... The circuit in Figure 14a generates both positive and negative 5V outputs with all components under 3mm height. The topology for the 5V output is a standard buck 9 5 converter. The –5V output uses a second inductor L2 diode D3 and output capacitor C6. The capacitor C4 LT1956/LT1956-5 MMSD914TI 5V, 1A ALTERNATE SUPPLY R1 15.4k ...
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... This is true even when the uncoupled inductors are sized (twice the value of inductance of the transformer) in order to keep ripple current comparable to the transformer solution single MMSD914TI C2 0.1 F BOOST LT1956 SHDN SYNC FB GND B0540W 2.2k ...
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... It differs from the standard approach in the way the IC chip derives its feedback signal because the LT1956 accepts only positive feedback sig- nals. The ground pin must be tied to the regulated negative output. A resistor divider to the FB pin, then provides the proper feedback voltage for the chip ...
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... – OUT P OUT For a 12V to –12V converter using the LT1956 with peak switch current of 1.5A and a catch diode of 0.63V CONT For a load current of 0.25A, this says that discontinuous mode can be used and the minimum inductor needed is ...
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... BSC 0 – 8 0.65 0.45 – 0.75 (.0256) BSC 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT MILLIMETERS (INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE LT1956/LT1956-5 4.90 – 5.10* (.193 – .201) 3.58 (.141) 16 1514 2.94 6.40 (.116) BSC ...
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... LT1956/LT1956-5 PACKAGE DESCRIPTIO .254 MIN .0165 .0015 RECOMMENDED SOLDER PAD LAYOUT .007 – .0098 (0.178 – 0.249) .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" ...