LT1507 Linear Technology, LT1507 Datasheet
LT1507
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LT1507 Summary of contents
Page 1
... A. The LT1507 can be externally synchronized from 570kHz to 1MHz with logic level inputs. The LT1507 fits into standard 8-pin SO and PDIP pack- ages. Temperature rise is kept to a minimum by the high efficiency design. Full cycle-by-cycle short-circuit protec- tion and thermal shutdown are provided ...
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... FB Pin Current (Adjustable Part) ............................. 1mA Sense Voltage (Fixed 3.3V Part) ................................ 5V Sync Pin Voltage ....................................................... 7V Operating Ambient Temperature Range LT1507C .................................................. LT1507I .............................................. – Max Operating Junction Temperature................... 125 C Storage Temperature Range ................. – 150 C Lead Temperature (Soldering, 10 sec).................. 300 C ELECTRICAL CHARACTERISTICS ...
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... J 40 3.8 5 2.3 2.38 2.46 0.15 0.37 0.70 0.25 0.45 0.70 1.5 2.2 580 1000 IN Feedback Pin Voltage and Current 2.44 2.43 VOLTAGE 2.42 CURRENT 2.41 2.40 – 1.25 –50 JUNCTION TEMPERATURE ( C) LT1507 UNITS kHz kHz kHz %/ kHz pin when voltage SW 2.0 1.5 1.0 0.5 0 125 100 LT1507 • TPC03 3 ...
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... JUNCTION TEMPERATURE ( C) LT1507 • TPC11 Shutdown Supply Current SHDN INPUT VOLTAGE (V) LT1507 • TPC06 Error Amplifier Transconductance 3000 200 PHASE 2500 150 GAIN 2000 100 1500 OUT OUT – ...
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... FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE 70% for conventional bipolar designs to greater than 85% for these new parts. V (Pin 2): Input Pin. The LT1507 is designed to operate IN with an input voltage between 4.5V and 15V. Under certain conditions, input voltage may be reduced down to 4V. Actual minimum operating voltage will always be higher than the output voltage ...
Page 6
... W BLOCK DIAGRAM The LT1507 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 ...
Page 7
... Figure 1. Block Diagram W U FEEDBACK PIN FUNCTIONS The feedback pin (FB or SENSE) on the LT1507 is used to set output voltage and also to 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 8
... Caution : clamping the feedback pin means that frequency shifting will also be defeated combination of high input voltage and dead shorted output may cause the LT1507 to lose control of current limit. The internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback pin when output voltage is low ...
Page 9
... 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 LT1507 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage and reduce core loss. Graphs in the Typical Performance Characteristics section show maximum output load current versus induc- tor size and input voltage ...
Page 10
... To get low ESR takes volume ; physi- cally larger capacitors have lower ESR. The ESR range needed for typical LT1507 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” ...
Page 11
... OUT )(ESR) (peak-to-peak) P– 3.3V ESR = 0 OUT P 500 P the LT1507 OUT A for OUT OUT A for LT1507 ) P 11 ...
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... LT1507 APPLICATIONS INFORMATION Example: with V = 3.3V 5V; OUT 3.3/5 = 0.67 OUT 1.75 – (0.5)(0.66) = 1.42A P Maximum load current would be equal to maximum switch current for an infinitely large inductor, but with finite inductor size, maximum load current is reduced by one half peak-to-peak inductor current. The following formula assumes continuous mode operation ...
Page 13
... Use the BAT85 Schottky diode for 3.3V applications where the anode is connected to the output. LAYOUT CONSIDERATIONS Suggested layout for the LT1507 is shown in Figure 3. The main concern for layout is to minimize the length of the INPUT D2 C2 ...
Page 14
... 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 LT1507, the input bypass capacitor should be rated at 0.75A ripple 5V current. Note however, that there are many secondary considerations in choosing the final ripple current rating ...
Page 15
... To calculate minimum operating input voltage, switch voltage loss and maximum duty cycle must be taken into account. With the LT1507 there is the additional consideration of proper operation of the boost circuit. The boost circuit allows the power switch to saturate for high efficiency, but it also sometimes results in a start-up or low current operating voltage that is 0 ...
Page 16
... ON time. Typically the ramp is gener- ated from a portion of the internal oscillator waveform. In the LT1507, the ramp is arranged to be zero until the oscillator waveform reaches about 40% of its final value. This minimizes the total amount of ramp added to switch current ...
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... THERMAL CALCULATIONS Power dissipation in the LT1507 chip comes from four value is S sources: switch DC loss, switch AC loss, boost circuit current and input quiescent current. The formulas below will be small S show how to calculate each of these losses. These formu- las assume continuous mode operation, so they should not be used for calculating efficiency at light load currents ...
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... The complete small-signal model is shown in Figure 9. R1 and R2 are the divider used to set output voltage. These are internal on the fixed voltage LT1507-3.3 with R1 = 1.8k and R2 = 5k. R POWER STAGE g = 1.8A/V ...
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... C = 3.3nF GAIN 40 100 PHASE –20 –50 0.01 0 100 100 FREQUENCY (kHz) LT1511 • F10 Figure 10. Overall Loop Phase and Gain 0.400* (10.160) MAX 0.255 0.015* (6.477 0.381) 0.015 (0.380) MIN LT1507 0695 19 ...
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... LT1507 PACKAGE DESCRIPTION 0.010 – 0.020 45 (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.016 – 0.050 0.406 – 1.270 * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" ...