LT1578 Linear Technology, LT1578 Datasheet

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... The topology is current mode for fast transient response and good loop stability. The LT1578 is a modified version of the LT1507 that has been optimized for noise sensitive appli- cations. It will operate over 15V input range. ...

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... FB Pin Voltage (Adjustable Part) ............................ 3.5V FB Pin Current (Adjustable Part) ............................ 1mA SYNC Pin Voltage ..................................................... 7V Operating Junction Temperature Range LT1578C ............................................... 125 C LT1578I ........................................... – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 C ELECTRICAL CHARACTERISTICS ture range, otherwise specifications are at T ...

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... To calculate gain and transconductance referred to the sense pin on the fixed voltage parts, divide values shown by the ratio 2.5/1.21. Switch Peak Current Limit 2.5 TYPICAL 2.0 MINIMUM 1.5 1.0 0 DUTY CYCLE (%) LT1578/LT1578-2.5 + 5V, switch open, unless otherwise noted. IN MIN TYP MAX 1.9 2 2.34 2.42 2.50 ...

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... LT1578/LT1578-2 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Pin Bias Current (V = Lockout Threshold) SHDN 4 AT 2.44V STANDBY THRESHOLD (CURRENT FLOWS OUT OF PIN –50 – 100 125 JUNCTION TEMPERATURE ( C) 1576 G04 Standby Thresholds 2.46 2.45 2.44 ON 2.43 STANDBY 2.42 2.41 2.40 –50 – 100 125 ...

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... OUT 1.0 0.8 0.6 0.4 0 INPUT VOLTAGE (V) 1578 G16 V C 1.0 0.8 0.6 0.4 0.2 0 1.00 1.25 1.50 –50 1576 G20 LT1578/LT1578-2.5 Minimum Input Voltage to Start with 3.3V Output 4.50 4.25 4.00 3.75 3.50 1 100 LOAD CURRENT (mA) Maximum Output Current 2.5V OUT 1.0 0.8 0.6 0.4 0 INPUT VOLTAGE (V) Pin Shutdown Threshold – ...

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... FET struc- ture. Efficiency improves from 75% for conventional bipo- lar designs to > 88% for the LT1578. GND (Pin 4): The GND pin connection needs consideration for two reasons. First, it acts as the reference for the regulated output, so load regulation will suffer if the “ ...

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... W BLOCK DIAGRAM The LT1578 is a constant frequency, current mode buck 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 ...

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... U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1578 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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... Current rating decreases with duty cycle because the LT1578 has internal slope compensation to prevent cur- rent mode subharmonic switching. For more details, read Application Note 19. The LT1578 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit ...

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... Choose a value in microhenries from the graphs of maximum load current and core loss. Choosing a small inductor may result in discontinuous mode operation at lighter loads, but the LT1578 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed core geometries IN like toroids ...

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... DO3308P-153 DO3316P-333 DO3316P-683 Pulse PE-53602 PE-53604 PE-53632 PE-53633 Gowanda SMP3316-152K SMP3316-332K SMP3316-682K Tor = Toroid SC = Semi-closed geometry Fer = Ferrite core material 52 = Type 52 powdered iron core material KM = Kool M LT1578/LT1578-2.5 SERIES CORE VALUE DC CORE RESIS- MATER- HEIGHT ( H) (Amps) TYPE TANCE( ) IAL 15 1.7 Tor 0.059 ...

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... To get low ESR takes volume , so physically smaller capacitors have high ESR. The ESR range for typical LT1578 applications is 0. typical output capacitor is an AVX type TPS, 100 F at 10V, with a guaranteed ESR less than 0.1 . This is a “D” size surface mount solid tantalum capacitor ...

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... Figure 3. LT1578 Ripple Voltage Waveform CATCH DIODE The suggested catch diode (D1 1N5818 Schottky, or its Motorola equivalent, MBR130 rated at 1A average forward current and 30V reverse voltage. Typical forward voltage is 0.42V at 1A. The diode conducts current only during switch off time ...

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... SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1578. Typically, UVLO is used in situations where the input supply is current limited , or has a relatively high source resistance particularly useful for input sup- plies with foldback current limiting. A switching regulator draws constant power from the source, so source current increases as source voltage drops ...

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... PC layout, you will see that it is irreducibly short. If you move the diode or input capacitor away from the LT1578, get your resumé in order. The other paths contain only some combination of DC and 200kHz triwave, so are much less critical. ...

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... OF OUTPUT CAPACITOR TO AVOID PARASITIC RESISTANCE AND INDUCTANCE (KELVIN CONNECTION) V OUT D2 C2 SYNC SHDN BOOST GND Figure 5. Suggested Layout for LT1578 SWITCH NODE L1 HIGH FREQUENCY V IN CIRCULATING PATH Figure 6. High Speed Switching Path MINIMIZE SIZE OF FEEDBACK PIN CONNECTIONS TO AVOID PICKUP TERMINATE ...

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... This switch off spike will also cause the SW node to go below ground. The LT1578 has special circuitry inside which mitigates this problem, but negative voltages over 1V lasting longer than 10ns should be avoided. Note that 100MHz oscilloscopes are barely fast enough to see the details of the falling edge overshoot in Figure 7 ...

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... It is common practice therefore to simply use the worst-case value and assume that RMS ripple current is one half of load current. At maximum output current of 1.5A for the LT1578, the input bypass capacitor should be rated at 0.75A ripple current. Note however, that there are many secondary considerations in choosing the final ripple current rating ...

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... By contrast, the LT1578 uses a “current mode” = 1A: architecture to help alleviate the phase shift created by the OUT inductor. The basic connections are shown in Figure 9. Figure 10 shows a Bode plot of the phase and gain of the power section of the LT1578, measured from the V LT1578/LT1578-2 ...

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... U APPLICATIONS INFORMATION the output. Gain is set by the 1.5A/V transconductance of the LT1578 power section and the effective complex impedance from output to ground. Gain rolls off smoothly above the 160Hz pole frequency set by the 100 F output capacitor. Phase drop is limited to about 85 . Phase recovers and gain levels off at the zero frequency ( 16kHz) set by capacitor ESR (0 ...

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... With pin 2 C LT1578/LT1578-2.5 is held to less C , the LT1578 will generally be well behaved . P-P is added to the loop, assuming that 200kHz ESR C ...

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... LT1578/LT1578-2 APPLICATIONS INFORMATION How Do I Test Loop Stability? The “standard” compensation for LT1578 is a 100pF capacitor for C , with While this compensation will C C work for most applications, the “optimum” value for loop compensation components depends, to various extents, on parameters which are not well controlled ...

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... LOAD greater than load current . This means that maximum load current will be significantly less than the LT1578’s 1.5A maximum switch current, even with large inductor values . The buck converter in comparison, delivers current to the output as a triangular wave superimposed level equal to load current, and load current can approach 1 ...

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... F P MAX does not take into account that maximum rated switch current ( the LT1578 is reduced slightly for duty P cycles above 50%. If duty cycle is expected to exceed 50% (input voltage less than output voltage), use the actual I value from the Electrical Characteristics table. ...

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... ESR of 0.1 . This is reasonable for AVX type TPS “D” or “E” size surface mount solid tantalum capacitors, but the final capacitor chosen must be looked at carefully for ESR characteristics OUT LT1578/LT1578-2 ...

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... LT1578/LT1578-2 APPLICATIONS INFORMATION Ripple Current in the Input and Output Capacitors Positive-to-negative converters have high ripple current in both the input and output capacitors. For long capacitor lifetime, the RMS value of this current must be less than the high frequency ripple current rating of the capacitor. ...

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... TYP 0.016 – 0.050 0.014 – 0.019 (0.355 – 0.483) TYP LT1578/LT1578-2.5 0.189 – 0.197* (4.801 – 5.004 0.150 – 0.157** (3.810 – 3.988 0.004 – 0.010 (0.101 – ...

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... L1B and C4 supply the – 5V rail. This reduces the current in L1A and changes L1B current waveform from square to triangular. For details on this circuit see Design Note 100. C2 0.33 F BOOST LT1578 SHDN FB V GND ...