LTC1736 LINER [Linear Technology], LTC1736 Datasheet

Page 1

... CMDSH 0.22 F INTV CC BOOST + 4 FDS6680A 2 PGND – + SENSE SENSE OSENSE 1000pF Figure 1. High Efficiency Step-Down Converter LTC1736 5-Bit Adjustable U ® 1736 is a synchronous step-down switching TM operation provides high effi F/50V CERAMIC SENSE 1.2 H 0.004 V OUT 1.35V TO 1.60V 12A + ...

Page 2

... Peak Driver Output Current <10 s (TG, BG) .............. 3A INTV Output Current ......................................... 50mA CC Operating Ambient Temperature Range LTC1736C ............................................... LTC1736I ............................................ – Junction Temperature (Note 2) ............................. 125 C Storage Temperature Range ................. – 150 C Lead Temperature (Soldering, 10 sec).................. 300 C ELECTRICAL CHARACTERISTICS temperature range, otherwise specifications are at T ...

Page 3

... • 110 C/ Note 3: The LTC1736 is tested in a feedback loop that servos V balance point for the error amplifier (V = 1.2V). ITH Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 5: Oscillator frequency is tested by measuring the C ...

Page 4

... LTC1736 W U TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current (3 Operating Modes) 100 EXTV OPEN CC 90 BURST 80 SYNC 70 CONT 1.6V OUT 0. 300kHz O 20 0.001 0.1 10 0.01 1 LOAD CURRENT (A) 1736 G01 Efficiency vs Input Voltage 100 EXTV OPEN 1.6V OUT 95 FIGURE 1 ...

Page 5

... FCB Pin Current vs Temperature 0.85V FCB –0.2 –0.4 –0.6 –0.8 –1.0 –40 – 110 135 TEMPERATURE ( C) 1736 G17 LTC1736 Maximum Current Sense Threshold vs Sense Common Mode Voltage 0.5 1 1.5 2 COMMON MODE VOLTAGE (V) 1736 G12 ITH RUN/SS 2 ...

Page 6

... LTC1736 W U TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency vs Temperature 300 C = 47pF OSC 290 280 270 260 250 –40 – 135 110 TEMPERATURE ( C) 1736 G19 Start-Up V OUT 1V/DIV V RUN/SS 5V/DIV I L 5A/DIV 1736 G22 5ms/DIV V = 15V 1.6V OUT R = 0.16 LOAD V OUT(RIPPLE) ...

Page 7

... CC TG (Pin 24): High Current Gate Drive for Top N-Channel MOSFET. This is the output of a floating driver with a voltage swing equal to INTV switch node voltage SW. LTC1736 voltages for the 32 combinations of digital or Float This switch closes and supplies V is higher than 4.7V. See EXTV ...

Page 8

... U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1736 uses a constant frequency, current mode step-down architecture. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the main current comparator I1 resets the RS latch. The peak inductor ...

Page 9

... This reduces peak inductor current to about 1/4 of its maximum value. Low Current Operation The LTC1736 has three low current modes controlled by the FCB pin. Burst Mode operation is selected when the FCB pin is above 0.8V (typically tied to INTV Burst Mode operation, if the error amplifier drives the I voltage below 0 ...

Page 10

... Functional Diagram) INTV /EXTV Power CC CC Power for the top and bottom MOSFET drivers and most of the internal circuitry of the LTC1736 is derived from the INTV pin. When the EXTV pin is left open, an internal CC CC 5.2V low dropout regulator supplies the INTV from V ...

Page 11

... Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, LTC1736 decreases with higher induc ...

Page 12

... Selection criteria for the power MOSFETs include the “ON” resistance R , reverse transfer capacitance C DS(ON) input voltage and maximum output current. When the LTC1736 is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by Main Switch Duty Cycle ...

Page 13

... The = output capaci- minimum capacitance to assure the inductors’ energy is OUT adequately absorbed is OUT requirement. where I is the change in load current. LTC1736 SENSE ) SENSE pin OPTI-LOOP compensation compo OUT ...

Page 14

... Efficiency Considerations section. The junction tempera- ture can be estimated by using the equations given in Note 2 of the Electrical Characteristics. For example, the LTC1736G is limited to less than 17mA from a 30V supply when not using the EXTV (17mA)(30V)(110 C/W) = 126 C ...

Page 15

... VID section normally connected to INTV other sources such as a 3.3V supply driven from another source, that source MUST be in the range of 2.7V to 5.5V and MUST be alive prior to enabling the LTC1736. LTC1736 pin and ground is a variable resistor, R1, FB OSENSE ...

Page 16

... Note represents codes without a defined output voltage as specified in Intel specifications. The LTC1736 interprets these codes as valid inputs and produces output voltages as follows: [01111] = 1.250V, [11111] = 0.900V Topside MOSFET Driver Supply (C VID0 V (V) An external bootstrap capacitor C OUT 0 2 ...

Page 17

... APPLICATIO S I FOR ATIO Soft-Start/Run Function The RUN/SS pin is a multipurpose pin that provides a soft- start function and a means to shut down the LTC1736. Soft-start reduces surge currents from V increasing the controller’s current limit I can also be used for power supply sequencing. ...

Page 18

... Under short-circuit conditions with very low duty cycle, the LTC1736 will begin cycle skipping in order to limit the short-circuit current. In this situation the bottom MOSFET will be conducting the peak current. The short-circuit ...

Page 19

... Minimum on-time t is the smallest amount of time ON(MIN) that the LTC1736 is capable of turning the top MOSFET on and off again determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on- ...

Page 20

... FCB pin, the FCB pin has a 0.17 A internal current source pulling the pin high. Remember to include this current when choosing resistor values R3 and R4. The internal LTC1736 oscillator can be synchronized to an external oscillator by clocking the FCB pin with a signal above 1.5V . When synchronized to an external fre- ...

Page 21

... Fast load transient response, limited board space and low cost are requirements of microprocessor power supplies. Active voltage positioning improves transient response pin not only allows and reduces the output capacitance required to power a TH microprocessor where a typical load step can be from 0.2A LTC1736 -C filter sets the dominant pole-zero C C pin waveforms TH ...

Page 22

... Resistors R1 and R5 force the input voltage offset that sets the output voltage according to the load current level. To select values for R1 and R5, first determine the amount of output deregulation allowed. The actual specification for a typical microprocessor allows the output to vary 0.112V. The LTC1736 output voltage R3 680k C8 0 ...

Page 23

... ITH output current: V ITH NOM ( Scale Factor The Thevenin equivalent of the gain limiting resistance value of 17.54k is made resistor R5 that sources current into the I 0 084 SGND. LTC1736 • 084 changes from 0.40V at light load to ITH ...

Page 24

... Note that the transient suppressor should not conduct during double-battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LTC1736 has a maximum input voltage of 36V, most applications will be limited to 30V by the MOSFET BV 1736 F07 ...

Page 25

... Figure 10. Check the following in your layout: and mini Are the signal and power grounds segregated? The LTC1736 PGND pin should tie to the GND plane close to V the input capacitor. The SGND pin should then connect 215 ns kHz ...

Page 26

... Boost node (BOOST) away from sensitive small-signal nodes, especially from the voltage and current sensing feedback pins. All of these nodes have very large and fast moving signals and therefore should be kept on the “output side” (Pins 13 to 24) of the LTC1736 and occupy minimum PC trace area. OSC 1 ...

Page 27

... DWG # 05-08-1640 – 8 0.65 (0.0256) BSC 0.25 – 0.38 (0.010 – 0.015) LTC1736 8.07 – 8.33* (0.318 – 0.328 7.65 – 7.90 (0.301 – 0.311) 1.73 – 1.99 (0.068 – 0.078) 0.05 – 0.21 (0.002 – 0.008) G24 SSOP 1098 ...

Page 28

... Converter with FCB Tied to PGOOD for CPU Power; Optimized for Output Voltages of 1.3V to 1.6V C OSC 47pF PGOOD 1 C OSC C 01 RUN/SS R 33k C 330pF 47pF 4 C2 FCB LTC1736 5 SGND 100k 6 INTV PGOOD CC 7 – SENSE 1000pF 8 + 47pF SENSE OSENSE 11 VID0 12 VID1 C ...