lt1738 Linear Technology Corporation, lt1738 Datasheet
lt1738
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lt1738 Summary of contents
Page 1
... N-channel MOSFET switch. Current and voltage slew rates can be independently set to optimize harmonic content of the switching waveforms vs efficiency. The LT1738 can reduce high frequency harmonic power by as much as 40dB with only minor losses in efficiency. The LT1738 utilizes a current mode architecture opti- mized for single switch topologies such as boost, flyback and Cuk ...
Page 2
... V = 0.9V FB REF C High Clamp Low Clamp 1.5V FB Outputs Drivers Disabled ORDER PART NUMBER 20 PGND NC 19 LT1738EG NC 18 LT1738IG VSL R 15 CSL SHDN GND 11 = 110 16.9k 16.9k and REF VSL CSL ...
Page 3
... Note 2: Supply current specification includes loads on each gate as in Figure 1a. Actual supply currents vary with operating frequency, operating voltages, V5 load, slew rates and type of external FET. Note 3: The LT1738E is guaranteed to meet performance specifications from Specifications from – 125 C are assured by The denotes the specifications which apply over the full operating = 25 C ...
Page 4
... LT1738 W U TYPICAL PERFOR A CE CHARACTERISTICS Feedback Voltage and Input Current vs Temperature 1.260 1.258 1.256 1.254 1.252 1.250 1.248 1.246 1.244 1.242 1.240 –50 – TEMPERATURE ( C) Feedback Overvoltage Shutdown vs Temperature 1.70 1.65 1.60 1.55 1.50 1.45 1.40 1.35 1.30 1.25 1.20 –50 – 100 125 150 TEMPERATURE ( C) 1738 G03 ...
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... TEMPERATURE ( C) 1738 G13 Soft-Start Current vs Temperature 9.5 SS VOLTAGE = 0.9V 9.3 9.1 8.9 8.7 8.5 8.3 8.1 7.9 7.7 7.5 –50 – 100 125 150 TEMPERATURE ( C) 1738 G16 LT1738 CS Pin to V Pin Transfer C Function 1 1.4 1.2 1.0 0.8 0.6 0.4 0 100 120 CS PIN VOLTAGE (mV) 1738 G11 Gate Drive Low Voltage vs Temperature 0 ...
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... LT1738 CTIO S Part Supply V5 (Pin 5): This pin provides a 5V output that can sink or source 10mA for use by external components. V5 source current comes from V . Sink current goes to GND must be greater than 6.5V in order for this voltage regulation. If this pin is used, a small capacitor (<1 F) may be placed on this pin to reduce noise ...
Page 7
... NFB. This pin should be left open if not used. If NFB is being used then overvoltage protection will occur at 0.44V below the NFB regulation point. pin cannot At NFB < –1.8 current sense blanking will be disabled. C LT1738 clamp voltage the V pin will connected from the S ...
Page 8
... LT1738 CTIO S V (Pin 12): The compensation pin is used for frequency C compensation and current limiting the output of the error amplifier and the input of the current comparator. Loop frequency compensation can be performed with an RC network connected from the V voltage proportional to the switch peak current. ...
Page 9
... Com- pliance with EMI standards will be an easier task and will require fewer external filtering components. The LT1738 uses an external N-channel MOSFET as the power switch. This allows the user to tailor the drive conditions to a wide range of voltages and currents. ...
Page 10
... LT1738 U OPERATIO The current slew feedback loop consists of the voltage across the external sense resistor, which is internally amplified and differentiated. The derivative is limited to a value set The current slew rate is thus inversely CSL proportional to both the value of sense resistor and R ...
Page 11
... The LT1738 provides control over two of the more impor- tant variables for controlling EMI with switching inductive loads: switch voltage slew rate and switch current slew rate ...
Page 12
... LT1738 U U APPLICATIO S I FOR ATIO Oscillator frequency is important for noise reduction in two ways. First the lower the oscillator frequency the lower the waveform’s harmonics, making it easier to filter them. Second the oscillator will control the placement of the output voltage harmonics which can aid in specific prob- lems where you might be trying to avoid a certain fre- quency bandwidth ...
Page 13
... When both FB and NFB pins are used, the LT1738 will act to prevent either output from going beyond its set output voltage. The highest output (lightest load) will dominate control of the regulator. This technique would prevent either output from going unregulated high at no load ...
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... LT1738 U U APPLICATIO S I FOR ATIO most of the control circuitry of the regulator. Note after the control circuitry powers on, gate driver activity will depend on the voltage of V with respect to the voltage on GCL the SHDN pin enables the internal regulator current will be sourced from the pin that can provide hysteresis for undervoltage lockout ...
Page 15
... V5 RMS sink current times 5V 3) The gate drive’s RMS discharge current times voltage. Because of the strong V to operate the LT1738 at as low always recommended that package temperature be measured in each application. The part has an internal thermal shutdown to minimize the chance of IC destruc- tion but this should not replace careful thermal design ...
Page 16
... LT1738 U U APPLICATIO S I FOR ATIO The following procedure can be used to handle these trade-offs: 1. Assume that the average inductor current for a boost converter is equal to load current times V decide whether the inductor must withstand continu- ous overload conditions. If average inductor current at maximum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 1 ...
Page 17
... ON capabilities of the MOSFET package as well as overall efficiency needs of the converter. The LT1738 can handle a large range of gate charges. However at very large charge stability may be affected. The power dissipation in the MOSFET depends on several factors. The primary element is I device is on ...
Page 18
... LT1738 U U APPLICATIO S I FOR ATIO First determine the maximum gate drive that you require. Typically you will want least 2V greater than the rated threshold. Higher voltages will lower the on resis- tance and increase efficiency. Be certain to check the maximum allowed gate voltage. Often this is 20V but for some logic threshold MOSFETs it is only 8V to 10V ...
Page 19
... Plastic SSOP (5.3mm) (Reference LTC DWG # 05-08-1640) 1.25 0.12 5.3 – 5.7 0.65 BSC 0 – 8 0.65 (.0256) BSC 6.90 – 7.50* (.272 – .295 ) 7.40 – 8.20 (.291 – .323 2.0 (.079) 0.05 0.22 – 0.38 (.002) (.009 – .015) G20 SSOP 0802 LT1738 1738fa 19 ...
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... INPUT FILTER IS REQUIRED TO ATTENUATE SWITCHING FREQUENCY HARMONICS AND PASS FCC CLASS B (LT1738 DOES NOT ATTENUATE THESE LOW FREQUENCY HARMONICS) MAIN ADVANTAGE WITH LT1738 IS IT MAKES SUPPRESSING THE HIGH FREQUENCY NOISE AND EMI EASY. THIS IS PARTICULARLY USEFUL FOR MEDICAL DEVICES BECAUSE THE AC LINE TO EARTH GND CAPS ON THE INPUT FILTER CAN BE ELIMINATED; ALLOWING THE DEVICE TO PASS THE EARTH GND LEAKAGE CURRENT MEDICAL SPECIFICATIONS. ...