LT1374 Linear Technology Corporation, LT1374 Datasheet
LT1374
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LT1374 Summary of contents
Page 1
... Efficiency is maintained over a wide output current range by using the output to bias the circuitry and by utilizing a supply boost capacitor to saturate the power switch. The LT1374 fits into standard 7-pin DD, TO-220 and fused lead SO-8 packages. Full cycle-by-cycle short-circuit protection and thermal shutdown are provided. Standard surface mount external parts are used, including the inductor and capacitors ...
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... FB Pin Current (Adjustable Part) ............................ 1mA SENSE Voltage (Fixed 5V Part) ................................. 7V SYNC Pin Voltage ..................................................... 7V Operating Junction Temperature Range LT1374C ............................................... 125 C LT1374I ........................................... – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 FRONT VIEW FB OR SENSE* ...
Page 3
... 5.6V FB SENSE Duty Cycle = 0 V Open 2. 4.4V SENSE 4.4V FB SENSE V Set to Give 50% Duty Cycle 25V, (5V V 32V for LT1374HV 10kHz BIAS BIAS V = 0V, V 25V 0V, V Open SHDN IN SW ...
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... LT1374 W U TYPICAL PERFORMANCE CHARACTERISTICS Switch Voltage Drop 500 450 125 C 400 350 25 C 300 250 – 200 150 100 SWITCH CURRENT (A) 1374 G18 Shutdown Pin Bias Current 500 CURRENT REQUIRED TO FORCE SHUTDOWN (FLOWS OUT OF PIN). AFTER SHUTDOWN, ...
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... Kool M PERMALLOY 0.01 = 125 CORE LOSS IS INDEPENDENT OF LOAD CURRENT UNTIL LOAD CURRENT FALLS LOW ENOUGH FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE 0.001 125 INDUCTANCE ( H) 1374 G11 LT1374 100 1000 1374 G12 1374 G15 1.2 0.8 ...
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... LT1374 PIN FUNCTIONS FB/SENSE: The feedback pin is the input to the error amplifier which is referenced to an internal 2.42V source. An external resistive divider is used to set the output voltage. The fixed voltage (-5) parts have the divider included on-chip and the FB pin is used as a SENSE pin, connected directly to the 5V output ...
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... PIN IS AVAILABLE ONLY ON THE S0-8 PACKAGE it much easier to frequency compensate the feedback loop and also gives much quicker transient response. Most of the circuitry of the LT1374 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 ...
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... U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1374 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 ...
Page 9
... Current rating decreases with duty cycle because the LT1374 has internal slope compensation to prevent cur- rent mode subharmonic switching. For more details, read Application Note 19. The LT1374 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit ...
Page 10
... 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 LT1374 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed core geometries like toroids ...
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... DT3316-332 DT3316-472 Pulse PE-53650 PE-53651 PE-53652 PE-53653 Dale IHSM-4825 IHSM-4825 IHSM-5832 IHSM-5832 IHSM-7832 Tor = Toroid SC = Semi-closed geometry Fer = Ferrite core material 52 = Type 52 powdered iron core material KM = Kool M LT1374 SERIES CORE VALUE DC CORE RESIS- MATER- HEIGHT ( H) (Amps) TYPE TANCE( ) IAL 2 4.1 Tor 0.011 ...
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... To get low ESR takes volume , so physically smaller capacitors have high ESR. The ESR range for typical LT1374 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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... P-P 20mV/DIV 0.5A/DIV 0.5 s/DIV Figure 3. LT1374 Ripple Voltage Waveform CATCH DIODE The suggested catch diode (D1 1N5821 Schottky, or its Motorola equivalent, MBR330 rated at 3A average forward current and 30V reverse voltage. Typical forward voltage is 0.5V at 3A. The diode conducts current only during switch off time ...
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... OUT SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1374. 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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... If you follow this path on the PC layout, you will see that it is irreducibly short. If you move the diode or input capacitor = 25k 114 114 380 k LT1374 15 ...
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... GND CONNECT TO PLACE FEEDTHROUGHS GROUND PLANE AROUND GND PIN FOR GOOD THERMAL CONDUCTIVITY KEEP FB AND V COMPONENTS C AWAY FROM HIGH FREQUENCY, HIGH CURRENT COMPONENTS MINIMIZE LT1374, C3, D1 LOOP Figure 5. Suggested Layout (Topside Only Shown) SWITCH NODE ...
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... U U APPLICATIONS INFORMATION away from the LT1374, get your resumé in order. The other paths contain only some combination of DC and 500kHz triwave, so are much less critical. PARASITIC RESONANCE Resonance or “ringing” may sometimes be seen on the switch node (see Figure 7). Very high frequency ringing following switch rise time is caused by switch/diode/input capacitor lead inductance and diode capacitance ...
Page 18
... 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 4.5A for the LT1374, the input bypass capacitor should be rated at 2.25A ripple current. Note however, that there are many secondary considerations in choosing the final ripple current rating ...
Page 19
... SYNC pin becomes operational synchronization is required, this pin should be connected to ground. THERMAL CALCULATIONS Power dissipation in the LT1374 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formu- las show how to calculate each of these losses. These ...
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... U U APPLICATIONS INFORMATION frequency. By contrast, the LT1374 uses a “current mode” architecture to help alleviate phase shift created by the 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 LT1374, measured from the V the output ...
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... None of this will show on a theoretical Bode plot because Bode is an amplitude insensitive analysis. Tests have shown that if ripple voltage on the V LT1374 will be well behaved . The formula below will give an estimate of V loop, assuming that 500kHz ...
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... LT1374 U U APPLICATIONS INFORMATION How Do I Test Loop Stability? The “standard” compensation for LT1374 is a 1.5nF capacitor for C , with While this compensation will C C work for most applications, the “optimum” value for loop compensation components depends, to various extent, on parameters which are not well controlled ...
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... Example: with 0.5V not take into account that maximum rated switch current ( the LT1374 is reduced slightly for duty cycles P above 50%. If duty cycle is expected to exceed 50% (input voltage less than output voltage), use the actual I from the Electrical Characteristics table. D1 ...
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... LT1374 U U APPLICATIONS INFORMATION Operating duty cycle OUT OUT F (This formula uses an average value for switch loss may be several percent in error.) With the conditions above This duty cycle is close enough to 50% that I assumed ...
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... Care should be used if diodes rated less than 3A are used, especially if continuous overload conditions must be tolerated. V OUT RMS OUT V IN Mode OUT IN OUT OUT OUT OUT Mode = L f LT1374 25 ...
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... LT1374 PACKAGE DESCRIPTION 0.060 0.256 (1.524) (6.502) 0.060 0.183 (1.524) (4.648) 0.075 (1.905) 0.300 (7.620) BOTTOM VIEW OF DD PAK HATCHED AREA IS SOLDER PLATED COPPER HEAT SINK 0.010 – 0.020 (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.016 – 0.050 0.406 – ...
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... LT1374 0.165 – 0.180 (4.191 – 4.572) 0.045 – 0.055 (1.143 – 1.397) 0.620 (15.75) TYP 0.095 – 0.115 (2.413 – ...
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... L1B and C4. At switch off, the energy stored in both 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 0.27 F BOOST LT1374-5 BIAS SHDN SENSE GND 470 ...