LT1767 Linear Technology, LT1767 Datasheet
LT1767
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LT1767 Summary of contents
Page 1
... Syn- chronization allows an external logic level signal to in- crease the internal oscillator from 1.4MHz to 2MHz. The LT1767 is available in an 8-pin MSOP fused leadframe package and a low thermal resistance exposed pad pack- age. Full cycle-by-cycle short-circuit protection and ther- mal shutdown are provided ...
Page 2
... Supply Current IN Shutdown Supply Current Feedback Voltage FB Input Current 2 U RATINGS (Note 1) SYNC Pin Current .................................................. 1mA Operating Junction Temperature Range (Note 2) LT1767E .......................................... – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 LT1767EMS8 LT1767EMS8-1.8 BOOST LT1767EMS8-2 ...
Page 3
... SYNC Pin Resistance Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1767E is guaranteed to meet performance specifications from 125 C. Specifications over the – 125 C operating junction temperature range are assured by design, characterization and correlation with statistical process controls ...
Page 4
... LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767 TYPICAL PERFOR A CE CHARACTERISTICS SHDN Threshold vs Temperature 1.40 1.38 1.36 1.34 1.32 1.30 –50 – 100 125 TEMPERATURE ( C) 1767 G04 Minimum Input Voltage for 2.5V Out 3.5 3.3 3.1 2.9 2.7 2.5 0.001 0.01 0.1 LOAD CURRENT (A) 1767 G07 Current Limit Foldback 2.0 1.5 SWITCH CURRENT 1.0 0.5 FB CURRENT 0 0 0.2 0.4 0.6 0.8 1 1.2 FEEDBACK VOLTAGE (V) 1767 G10 ...
Page 5
... Keep the ground path short between the GND pin and the load and use a ground plane LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 when possible. Keep the path between the input bypass and the GND pin short. The GND pin of the MS8 package is directly attached to the internal tab ...
Page 6
... LT1767-2.5/LT1767-3.3/LT1767-5 W BLOCK DIAGRAM The LT1767 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
... 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 LT1767 and force the switching current into a tight local loop, thereby minimizing EMI. The RMS ripple current can be calculated from: ...
Page 8
... Choose a value in microhenries from the graphs of maximum load current. Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1767 is designed to work well in either mode. Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions ...
Page 9
... If available, an independent supply can be used with a local 0.080 3.5 bypass capacitor. 0.058 1.8 0.071 1.8 A 0.1 F boost capacitor is recommended for most appli- 0.048 2.8 cations. Almost any type of film or ceramic capacitor is LT1767/LT1767-1. OUT IN OUT V IN above V to drive the output OUT IN ...
Page 10
... SHUTDOWN AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1767. 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 ...
Page 11
... Similar treatment of this pin will result in lower die temperatures. THERMAL CALCULATIONS Power dissipation in the LT1767 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formulas show how to calculate each of these losses. ...
Page 12
... C3, D1 LOOP Figure 6. Typical Application and Suggested Layout (Topside Only Shown) Total power dissipation is 0.34 + 0.05 + 0.01 = 0.4W. Thermal resistance for LT1767 package is influenced by the presence of internal or backside planes. With a full plane under the package, thermal resistance for the exposed pad package will be about 40 C/W. No plane will increase resistance to about 150 C/W ...
Page 13
... The basic regulator loop is shown in Figure 7, with both tantalum and ceramic capacitor equivalent cir- cuits. The LT1767 can be considered as two g error amplifier and the power stage. Figure 8 shows the overall loop response with a 330pF V capacitor and a typical 100 F tantalum output capacitor ...
Page 14
... CONVERTER WITH BACKUP OUTPUT REGULATOR In systems with a primary and backup supply, for ex- ample, a battery powered device with a wall adapter input, the output of the LT1767 can be held up by the backup supply with its input disconnected. In this condition, the SW pin will source current into the V is held at ground, only the shut down current will be pulled via the SW pin from the second supply ...
Page 15
... BSC LT1767/LT1767-1.8/ 3.3V, 1A ALTERNATE SUPPLY 2.2 F 1767 F09 D2 CMDSH-3 C2 0.1 F L1A BOOST LT1767-5 SHDN FB SYNC GND 2 16V 330pF D1 CERAMIC C4 2.2 F 100 F L1B* 16V 10V TANT CERAMIC D3 Figure 11. Dual Output SEPIC Converter 0.118 0.004* (3 ...
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... LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 PACKAGE DESCRIPTION 0.889 0.127 2.794 0.102 (.035 .005) (.110 .004) 5.23 2.083 0.102 3.2 – 3.45 (.206) (.126 – .136) (.082 .004) MIN 0.65 0.42 0.04 (.0165 .0015) (.0256) TYP BSC RECOMMENDED SOLDER PAD LAYOUT NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. ...