LT1940 Linear Technology, LT1940 Datasheet
LT1940
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LT1940 Summary of contents
Page 1
... Constant frequency and ce- ramic capacitors combine to produce low, predictable output ripple voltage. With its wide input range of 3.6V to 25V, the LT1940 regulates a wide variety of power sources, from 4-cell batteries and 5V logic rails to unregulated wall transformers, lead acid batteries and distributed-power supplies ...
Page 2
... 25V 1. 1. 1MHz Note ORDER PART NUMBER 16 FB1 LT1940EFE 14 PG1 13 RUN/SS1 12 RUN/SS2 11 PG2 FB2 = 45 C/W MIN TYP MAX UNITS 3.4 3.6 3.8 4 1.230 1.250 1.270 1.225 1.250 1.270 1.215 1 ...
Page 3
... PG Voltage Output Low PG Pin Leakage Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1940E is guaranteed to meet performance specifications from Specifications over the – operating W U TYPICAL PERFOR A CE CHARACTERISTICS Efficiency ...
Page 4
... LT1940 W U TYPICAL PERFOR A CE CHARACTERISTICS Boost Pin Current 0.5 1.0 1.5 2.0 SWITCH CURRENT (A) 1940 G07 Frequency vs Temperature 1.3 1.2 1.1 1.0 0.9 –50 – 100 125 TEMPERATURE ( C) 1940 G10 4 Current Limit vs Duty Cycle 3.0 2.5 TYPICAL 2.0 MINIMUM 1.5 1.0 0 100 DUTY CYCLE (%) 1940 G08 Frequency Foldback 1 ...
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... IN IN LT1940’s internal regulator and to the internal power switches. These pins must be tied to the same source, and must be locally bypassed. FB1, FB2 (Pins 9, 16): The LT1940 regulates each feed- back pin to 1.25V. Connect the feedback resistor divider taps to these pins ...
Page 6
... RUN/ GND Figure 2. Block Diagram of the LT1940 with Associated External Components (One of Two Switching Regulators Shown) The LT1940 is a dual, constant frequency, current mode buck regulator with internal 2A power switches. The two regulators share common circuitry including input source, voltage reference and oscillator, but are otherwise inde- pendent. This section describes the operation of the LT1940 ...
Page 7
... A power good comparator trips when the FB pin is at 90% of its regulated value. The PG output is an open collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT1940 is enabled (either RUN/SS pin is 0.8V, and an C pin is high) and V ...
Page 8
... When the switch is off, the inductor sees the output voltage plus the catch diode drop. This gives the peak-to- peak ripple current in the inductor – DC)(V L where f is the switching frequency of the LT1940 and L is the value of the inductor. The peak inductor and switch current SWPK To maintain output regulation, this peak current must be less than the LT1940’ ...
Page 9
... LQH4C3R3M04 3.3 0.80 Input Capacitor Selection Bypass the input of the LT1940 circuit with a 4 higher ceramic capacitor of X7R or X5R type. A lower value or a less expensive Y5V type can be used if there is additional bypassing provided by bulk electrolytic or tantalum capacitors. The following paragraphs describe the input capacitor considerations in more detail. ...
Page 10
... The output capacitor filters the inductor current to gener- ate an output with low voltage ripple. It also stores energy in order satisfy transient loads and to stabilize the LT1940’s control loop. Because the LT1940 operates at a high frequency, you don’t need much output capacitance. Also, the current mode control loop doesn’ ...
Page 11
... Figure 3. Generating the Boost Voltage LT1940 D2 C3 BOOST LT1940 OUT IN GND V – BOOST SW IN MAX V 2V BOOST IN (3b) D2 BOOST LT1940 OUT GND 1940 F03 MAX V – BOOST SW IN2 MAX V V BOOST IN2 MINIMUM VALUE FOR IN2 = IN (3d) 1940i ...
Page 12
... Frequency Compensation The LT1940 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT1940 does not require the ESR of the output capacitor for stability so you are free to use ceramic capacitors to achieve low output ripple and small circuit size ...
Page 13
... If the inductor is chosen so that it won’t saturate exces- sively, the LT1940 will tolerate a shorted output. There is pin, limiting the peak another situation to consider in systems where the output will be held high when the input to the LT1940 is absent. /10,000, OUT If the V and one of the RUN/SS pins are allowed to float, IN then the LT1940’ ...
Page 14
... U Thermal Considerations The PCB must also provide heat sinking to keep the LT1940 cool. The exposed metal on the bottom of the package must be soldered to a ground plane. This ground should be tied to other copper layers below with thermal vias; these layers will spread the heat dissipated by the LT1940 ...
Page 15
... OUT1 Figure 9. A Good PCB Layout Ensures Proper Low EMI Operation Single, Low-Ripple 2.8A Output The LT1940 can generate a single, low-ripple 2.8A output if the outputs of the two switching regulators are tied together and share a single output capacitor. By tying the two FB pins together and the two V C channels will share the load current ...
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... H OUT1 1.8V 1.4A 10.0k C1 22. C1: TAIYO YUDEN JMK316BJ226ML C2: TAIYO YUDEN JMK316BJ106ML C3: TAIYO YUDEN EMK316BJ475ML POWER GOOD 16 3.3V and 1.8V Outputs with Sequencing 4.7 F BOOST1 BOOST2 LT1940 0.1 F 0.1 F SW1 SW2 D1 FB1 FB2 20k 15k RUN/SS1 PG1 RUN/SS2 PG2 220pF GND 1nF D1, D2: MICROSEMI UPS120 ...
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... LT1940 0.1 F SW1 SW2 D1 FB1 FB2 10.0k PG1 PG2 RUN/SS1 RUN/SS2 GND 1nF 1nF D1, D2: MICROSEMI UPS140 OR ON SEMI MBRM140 D3, D4: CENTRAL CMDSH-3 L1: SUMIDA CDRH4D28-3R3 L2: SUMIDA CDRH4D28-4R7 LT1940 D4 OUT2 5V 1.2A L2 0 30. 20k 10. 10V 100pF 100k 5 GOOD 1940 TA03 1940i 17 ...
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... C1, C2, C4: TAIYO YUDEN JMK316BJ106ML C3: TAIYO YUDEN TMK325BJ475ML D1, D2: MICROSEMI UPS140 OR ON SEMI MBRM140 D3: BAT-54A Low Ripple, Low Profile 12V to 3.3V/2.4A Maximum Height = 2.1mm V IN RUN/SS1 BOOST1 1nF 0.1 F RUN/SS2 PG1 SW1 100k D1 PG2 LT1940 V C1 6.8k 680pF BOOST2 V C2 0.1 F FB1 330pF SW2 FB2 16.5k GND D2 10.0k OUT3 – ...
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... EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 0.45 0.05 1.05 0. – 8 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) (INCHES) LT1940 4.95 – 5.05* 3.0 (.118 3.0 6.25 – 6.50 (.118) (.246 – .256) 1.15 (.0453) MAX 0.05 – 0.15 (.002 – .006) FE16 TSSOP 1101 1940i 19 ...
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... McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com Low Ripple, Low Profile 5V to 3.3V/2.4A Maximum Height = 1.4mm 0. D3B IN BOOST1 RUN/SS1 1nF 0.1 F RUN/SS2 SW1 PG1 100k PG2 LT1940 V C1 D4B 4.7k BOOST2 V C2 FB1 0.1 F 330pF SW2 FB2 16.5k GND 10k COMMENTS 6-Lead ThinSOT Dual Synchronous Controller 60V Input, 0 ...