LT1977 Linear Technology, LT1977 Datasheet
LT1977
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LT1977 Summary of contents
Page 1
... The device also has a power good flag with a programmable threshold and time-out and thermal shutdown protection. The LT1977 is available in a 16-pin TSSOP package with exposed pad leadframe for low thermal resistance. The LT1976, a 200kHz reduced switch frequency version of the LT1977, is also available ...
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... BOOST Pin Voltage ................................................. 68V SYNC PGFB, FB ................................................ 6V SS Operating JunctionTemperature Range LT1977EFE (Note 2) ........................ – 40°C to 125°C LT1977IFE (Note 2) ......................... – 40°C to 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ELECTRICAL CHARACTERISTICS temperature range, otherwise specifications are at T unless otherwise noted ...
Page 3
... Note 4: Supply input current is the quiescent current drawn by the input pin. Its typical value depends on the voltage on the BIAS pin and operating state of the LT1977. With the BIAS pin at 0V, all of the quiescent current required to operate the LT1977 will be provided by the V BIAS voltage above its minimum input voltage, a portion of the total quiescent current will be supplied by the BIAS pin ...
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... LT1977 W U TYPICAL PERFOR A CE CHARACTERISTICS FB Voltage 1.30 1.29 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.20 –50 – 100 125 TEMPERATURE (°C) 1977 G01 SHDN Pin Current 5 25°C J 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1 SHDN VOLTAGE (V) 1977 G04 Bias Sleep Current 200 180 160 140 120 100 – ...
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... LOAD CURRENT = 0.5A 300 250 LOAD CURRENT = 1A 200 150 100 50 0 – 125 –25 100 TEMPERATURE (°C) 1977 G21 LT1977 Frequency Foldback Percentage 100 0.25 0.5 0.75 1 1.25 FB PIN VOLTAGE (V) 1977 G12 Minimum Input Voltage 8 ...
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... LT1977 W U TYPICAL PERFOR A CE CHARACTERISTICS Burst Mode Operation V OUT 20mV/DIV I SW 500mA/DIV V = 12V 5ms/DIV 3.3V OUT I = 100µ Load 1A Step Response V OUT 50mV/DIV I OUT 500mA/DIV V = 12V 500µs/DIV 3.3V OUT C = 100µF OUT I = 0mA CTIO S NC (Pins 1, 3, 5): No Connection ...
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... C high impedance when the clamp is active. The PG pin has a typical sink capability of 200µA. See the Power Good section in Applications Information for details. sits C LT1977 pin to ground C pin. See the Feedback section When PGFB ...
Page 8
... LC resonant fre- quency. This makes it much easier to frequency compen- sate the feedback loop and also gives much quicker tran- sient response. Most of the circuitry of the LT1977 operates from an internal 2.4V bias line. The bias regulator normally draws + CURRENT COMP – ...
Page 9
... LT1977. Typical minimum and maximum duty cycles are 6% to 95% for the LT1976 and 15% to 90% for the LT1977. Both parts will regulate input voltage of 60V but the LT1977 will transistion into pulse skipping/Burst Mode operation when the input voltage is above 30V for a 5V output ...
Page 10
... Frequency Foldback graph). In addition, if the current in 1977 F02 the switch exceeds 1.5 times the current limitations speci- fied by the V LT1977 will skip the next switch cycle. As the feedback voltage rises, the switching frequency increases to 500kHz with 0.95V on the FB pin. During frequency foldback, OUTPUT ...
Page 11
... 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 LT1977 and force the switching current into a tight local loop, thereby minimiz- ing EMI. The RMS ripple current can be calculated from: ...
Page 12
... MAXIMUM OUTPUT LOAD CURRENT Maximum load current for a buck converter is limited by the maximum switch current rating (I specified current rating for the LT1977 is 1.5A. Unlike most current mode converters, the LT1977 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% ...
Page 13
... Choose a value in microhenries such that the maximum load current plus half the ripple current is less than the minimum peak switch current (I inductor with lighter loads may result in discontinuous ) mode of operation, but the LT1977 is designed to work )( ) e well in either mode. A Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions ...
Page 14
... W U Short-Circuit Considerations DCR HEIGHT The LT1977 is a current mode controller. It uses the V (Ohms) (mm) node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as 0.111 5.0 this peak current is reached. The internal clamp on the V ...
Page 15
... V OUT 1V/DIV capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode opera- tion, the LT1977 delivers short bursts of current to the + V )] ratios > output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. In addition, V are reduced to typically 45µ ...
Page 16
... F05 perature can potentially exceed the magnitude of the IN LT1977 supply current. Low forward drop is critical for high current efficiency since the loss is proportional to C forward drop. These requirements result in the use of a Schottky type diode ...
Page 17
... SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT The SHDN pin on the LT1977 controls the operation of the IC. When the voltage on the SHDN pin is below the 1.2V shutdown threshold the LT1977 is placed in a “zero” supply current state. Driving the SHDN pin above the shutdown threshold enables normal operation. The SHDN pin has an internal sink current of 3µ ...
Page 18
... LT1977 U U APPLICATIO S I FOR ATIO In addition to the shutdown feature, the LT1977 has an undervoltage lockout function. When the input voltage is below 2.4V, switching will be disabled. The undervoltage lockout threshold doesn’t have any hysteresis and is mainly used to insure that all internal voltages are at the correct level before switching is enabled ...
Page 19
... PG inactive, resulting in a non Power Good cycle when SHDN is taken above its threshold. Figure 9 shows the power good operation with PGFB connected to FB and the capacitance on C several different configurations for the LT1977 Power Good circuitry. LAYOUT CONSIDERATIONS 1977 F09 ...
Page 20
... This is the best thermal path for heat out of the package. Reducing the thermal resistance from Pin 8 and exposed pad onto the board will reduce die temperature and in- crease the power capability of the LT1977. This is achieved by providing as much copper area as possible around the exposed pad. Adding multiple solder filled feedthroughs under and around this pad to an internal ground plane will also help ...
Page 21
... V = Schottky diode forward drop switching frequency OSC A potential control problem arises if the LT1977 is called upon to produce an on time shorter than it is able to produce. Feedback loop action will lower then reduce the V control voltage to the point where some sort of cycle- C skipping or Burst Mode behavior is exhibited. ...
Page 22
... The LT1977 uses current mode control. This alleviates many of the phase shift problems associated with the inductor. The basic regulator loop is shown in Figure 12. The LT1977 can be considered as two g amplifier and the power stage. Figure 13 shows the overall loop response with a 330pF V capacitor and a typical 100µ ...
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... 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 LT1977 4.90 – 5.10* (.193 – .201) 3.58 (.141) 16 1514 6.40 2.94 (.252) ( ...
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... LT1977 RELATED PARTS PART NUMBER DESCRIPTION LT1074/LT1074HV 4. 100kHz, High Efficiency Step-Down DC/DC Converters OUT LT1076/LT1076HV 1. 100kHz, High Efficiency Step-Down DC/DC Converters OUT LT1676 60V, 440mA (I ), 100kHz, High Efficiency Step-Down DC/DC OUT Converter LT1765 25V 1.25MHz, High Efficiency Step-Down DC/DC OUT Converter LT1766 60V, 1. 200kHz, High Efficiency Step-Down DC/DC ...