lt1976 Linear Technology Corporation, lt1976 Datasheet
lt1976
Available stocks
Related parts for lt1976
lt1976 Summary of contents
Page 1
... Burst Mode operation at low currents, utilizing the output to bias the internal circuitry, and by using a supply boost capacitor to fully saturate the power switch. The LT1976B does not shift into Burst Mode operation at low currents, eliminating low frequency out- put ripple at the expense of efficiency. Patented circuitry maintains peak switch current over the full duty cycle range ...
Page 2
... SYNC PGFB, FB ................................................ 6V SS Operating JunctionTemperature Range LT1976EFE/LT1976BEFE (Note 2) ... – 40°C to 125°C LT1976IFE/LT1976BIFE (Note 2) ..... – 40°C to 125°C LT1976HFE (Note 2) ........................ – 40°C to 140°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ELECTRICAL CHARACTERISTICS operating temperature range, otherwise specifications are at T FB/PGFB = 1 ...
Page 3
... PARAMETER V SHDN Threshold SHDN I SHDN Input Current SHDN Minimum Input Voltage (Note 3) I Supply Shutdown Current VINS Supply Sleep Current (Note 4) (LT1976) I Supply Quiescent Current VIN Minimum BIAS Voltage (Note 5) I BIAS Sleep Current (Note 4) BIASS I BIAS Quiescent Current BIAS Minimum Boost Voltage (Note 6) ...
Page 4
... 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 LT1976. With the BIAS pin at 0V, all of the quiescent current required to operate the LT1976 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 ...
Page 5
... W U TYPICAL PERFOR A CE CHARACTERISTICS LT1976 Efficiency and Power Loss vs Load Current 100 EFFICIENCY 75 5V 3.3V 50 TYPICAL POWER LOSS 25 0 0.1 1 100 1000 10000 10 LOAD CURRENT (mA) 1976 TA02 Oscillator Frequency 250 240 230 220 210 200 190 180 170 160 150 50 125 150 – ...
Page 6
... T = 125°C J 250 200 T = 25°C J 150 100 T = –50° 0.5 0.7 0.9 1.1 1.3 1.5 LOAD CURRENT (A) 1976 G13 LT1976 Burst Mode Threshold vs Input Voltage 200 V = 3.3V OUT 180 L = 33μ 100μF OUT 160 140 120 100 ...
Page 7
... LOAD CURRENT (mA) LT1976 Burst Mode Operation V OUT 50mV/DIV I SW 100mA/DIV 12V TIME (5ms/DIV 3.3V OUT I = 100μ LT1976 No Load 1A Step Response V OUT 100mV/DIV 1A I OUT 500mA/DIV 12V TIME (1ms/DIV 3.3V OUT C = 47μF OUT 1976 G22 ...
Page 8
... CTIO S NC (Pins 1, 3, 5): No Connection. Pins are electrically isolated from the LT1976. They may be con- nected to PCB traces to aid in PCB layout. SW (Pin 2): The SW pin is the emitter of the on-chip power NPN switch. This pin is driven up to the input pin voltage during switch on time ...
Page 9
... Driving the V pin to ground will disable switch- C ing and also place the LT1976 into sleep mode. FB (Pin 12): The feedback pin is used to determine the output voltage using an external voltage divider from the output that generates 1.25V at the FB pin . When the FB pin drops below 0 ...
Page 10
... 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 LT1976 operates from an internal 2.4V bias line. The bias regulator normally draws + CURRENT COMP – ...
Page 11
... The only difference between the LT1976 and the LT1976B is that the LT1976B does not shift into burst mode in light load situations, eliminating low frequency output ripple at the expense of light load efficiency. ...
Page 12
... U U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1976 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 frequency foldback and soft-start features ...
Page 13
... 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 LT1976 and force the switching current into a tight local loop, thereby minimiz- ing EMI. The RMS ripple current can be calculated from: ...
Page 14
... C = 27pF. 0.7 to 1.1 FB 0.5 OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT1976. Ripple voltage is determined by the impedance of the output capacitor and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is P-P ...
Page 15
... Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current . OUT seen by the LT1976 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage and reduce core ) V I ...
Page 16
... U APPLICATIO S I FOR ATIO 1. Choose a value in microhenries from the graph of maximum load current. Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1976 is designed to work well in either mode. Table 4. Inductor Selection Criteria VENDOR/ PART NUMBER VALUE (μH) ...
Page 17
... U U APPLICATIO S I FOR ATIO Short-Circuit Considerations The LT1976 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 2.2V, then acts as an output switch peak current limit ...
Page 18
... IN duced to typically 45μA and 125μA respectively during the sleep time. As the load current decreases towards a no load condition, the percentage of time that the LT1976 operates in sleep mode increases and the average input current is greatly reduced resulting in higher efficiency. The minimum average input current depends on the V ...
Page 19
... V loads. If the generated BOOST voltage dissipates too 821μA 83.7% much power at maximum load, the BOOST voltage the LT1976 sees can be reduced by placing a Zener diode in 1088μA 84.5% series with the BOOST diode (Figure 7a option). LT1976/LT1976B current into the diode in an ...
Page 20
... W U SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT The SHDN pin on the LT1976 controls the operation of the V OUT IC. When the voltage on the SHDN pin is below the 1.2V shutdown threshold the LT1976 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 21
... SYNCHRONIZING Oscillator synchronization to an external input is achieved by connecting a TTL logic-compatible square wave with a duty cycle between 20% and 80% to the LT1976 SYNC pin. The synchronizing range is equal to initial operating frequency up to 700kHz. This means that minimum practical sync frequency is equal to the worst-case high self-oscillating frequency (230kHz), not the typical oper- ating frequency of 200kHz ...
Page 22
... Burst Mode operation. If erratic operation occurs during these conditions a small filter capacitor from the PGOOD pin to ground will ensure proper operation. Figure 10 shows several different con- 1976 F09 figurations for the LT1976 Power Good circuitry 200k ...
Page 23
... U LT1976 switch. When operating at higher currents and input voltages, with poor layout, this spike can generate voltages across the LT1976 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. ...
Page 24
... 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 LT1976. 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 25
... V = Schottky diode forward drop switching frequency OSC A potential controllability problem arises if the LT1976 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 26
... The LT1976 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 LT1976 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 ...
Page 27
... 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 LT1976/LT1976B LT1976B Efficiency and Power Loss vs Load Current 100 EFFICIENCY 75 5V 3.3V 50 TYPICAL POWER LOSS ...
Page 28
... LT1976/LT1976B 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 ...