LT1374C Linear Technology, LT1374C Datasheet

Page 1

... There is the optional function of shutdown or synchronization. A shutdown signal reduces supply current Synchronization allows an exter- nal logic level signal to increase the internal oscillator from 580kHz to 1MHz. , LTC and LT are registered trademarks of Linear Technology Corporation. D2 1N914 C2 0.27 F L1** ...

Page 2

... U RATINGS (Note 1) FB Pin Current (Adjustable Part) ............................ 1mA SENSE Voltage (Fixed 5V Part) ................................. 7V SYNC Pin Voltage ..................................................... 7V Operating Junction Temperature Range LT1374C ............................................... 125 C LT1374I ........................................... – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 FRONT VIEW FB OR SENSE* ...

Page 3

ELECTRICAL CHARACTERISTICS ture range, otherwise specifications are at T PARAMETER Feedback Input Bias Current Error Amplifier Voltage Gain Error Amplifier Transconductance V Pin to Switch Current Transconductance C Error Amplifier Source Current Error Amplifier Sink Current V Pin Switching Threshold ...

Page 4

LT1374 W U TYPICAL PERFORMANCE CHARACTERISTICS Switch Voltage Drop 500 450 125 C 400 350 25 C 300 250 – 200 150 100 SWITCH CURRENT (A) 1374 G18 Shutdown Pin ...

Page 5

W U TYPICAL PERFORMANCE CHARACTERISTICS Frequency Foldback 500 400 SWITCHING FREQUENCY 300 200 100 FEEDBACK PIN CURRENT 0 0 0.5 1.0 1.5 2.0 2.5 FEEDBACK PIN VOLTAGE (V) 1374 G10 Maximum Load Current 10V OUT 4.5 V ...

Page 6

LT1374 PIN FUNCTIONS FB/SENSE: The feedback pin is the input to the error amplifier which is referenced to an internal 2.42V source. An external resistive divider is used to set the output voltage. The fixed voltage (-5) ...

Page 7

W BLOCK DIAGRAM The LT1374 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 ...

Page 8

LT1374 U U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1374 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output ...

Page 9

U U APPLICATIONS INFORMATION time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 500kHz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 1V (see ...

Page 10

LT1374 U U APPLICATIONS INFORMATION • 500 10 • Note that there is less load current available at the ...

Page 11

... After making an initial choice, consider the secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology’s applica- tions department if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest devel- opments in low profile, surface mounting, etc ...

Page 12

... ESL before ESR becomes effective. They are appropriate for input bypassing because of their high ripple current ratings and tolerance of turn-on surges. ) Ripple Current (A) Linear Technology plans to issue a design note on the use 0.7 to 1.1 of ceramic capacitors in the near future. 0.4 OUTPUT RIPPLE VOLTAGE ...

Page 13

U U APPLICATIONS INFORMATION Peak-to-peak output ripple voltage is the sum of a triwave created by peak-to-peak ripple current times ESR, and a square wave created by parasitic inductance (ESL) and ripple current slew rate. Capacitive reactance is assumed to ...

Page 14

LT1374 U U APPLICATIONS INFORMATION WARNING! Peak voltage on the BOOST pin is the sum of unregulated input voltage plus the voltage across the boost capacitor. This normally means that peak BOOST pin voltage is equal to input voltage plus ...

Page 15

U U APPLICATIONS INFORMATION prevents the regulator from operating at source voltages where these problems might occur. Threshold voltage for lockout is about 2.38V, slightly less than the internal 2.42V reference voltage. A 3.5 A bias current flows out of ...

Page 16

LT1374 U U APPLICATIONS INFORMATION V IN GND CONNECT TO PLACE FEEDTHROUGHS GROUND PLANE AROUND GND PIN FOR GOOD THERMAL CONDUCTIVITY KEEP FB AND V COMPONENTS C AWAY FROM HIGH FREQUENCY, HIGH CURRENT COMPONENTS MINIMIZE LT1374, C3, ...

Page 17

U U APPLICATIONS INFORMATION away from the LT1374, get your resumé in order. The other paths contain only some combination of DC and 500kHz triwave, so are much less critical. PARASITIC RESONANCE Resonance or “ringing” may sometimes be seen on ...

Page 18

LT1374 U U APPLICATIONS INFORMATION two capacitors to obtain the required ripple rating. Both capacitors must be of the same value and manufacturer to guarantee power sharing. The actual value of the capacitor in microfarads is not particularly important because ...

Page 19

U U APPLICATIONS INFORMATION different cause of subharmonic switching before assum- ing that the cause is insufficient slope compensation. Application Note 19 has more details on the theory of slope compensation. At power-up, when V is being clamped by the ...

Page 20

LT1374 U U APPLICATIONS INFORMATION frequency. By contrast, the LT1374 uses a “current mode” architecture to help alleviate phase shift created by the inductor. The basic connections are shown in Figure 9. Figure 10 shows a Bode plot of the ...

Page 21

U U APPLICATIONS INFORMATION Analog experts will note that around 4.4kHz, phase dips very close to the zero phase margin line. This is typical of switching regulators, especially those that operate over a wide range of loads. This region of ...

Page 22

LT1374 U U APPLICATIONS INFORMATION How Do I Test Loop Stability? The “standard” compensation for LT1374 is a 1.5nF capacitor for C , with While this compensation will C C work for most applications, the “optimum” value ...

Page 23

U U APPLICATIONS INFORMATION If everything looks OK, I use a heat gun and cold spray on the circuit (especially the output capacitor) to bring out any temperature-dependent characteristics. Keep in mind that this procedure does not take initial component ...

Page 24

LT1374 U U APPLICATIONS INFORMATION Operating duty cycle OUT OUT F (This formula uses an average value for switch loss may be several percent in error.) With the ...

Page 25

U U APPLICATIONS INFORMATION Minimum inductor continuous mode OUT L MIN OUT P OUT For the example above, with maximum load current of 1A ...

Page 26

LT1374 PACKAGE DESCRIPTION 0.060 0.256 (1.524) (6.502) 0.060 0.183 (1.524) (4.648) 0.075 (1.905) 0.300 (7.620) BOTTOM VIEW OF DD PAK HATCHED AREA IS SOLDER PLATED COPPER HEAT SINK 0.010 – 0.020 (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) ...

Page 27

... Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U Dimensions in inches (millimeters) unless otherwise noted ...

Page 28

... High Efficiency DC/DC Converter LT1372/LT1377 500kHz and 1MHz High Efficiency 1.5A Switching Regulators LTC1735 High Efficiency Step-Down Converter Burst Mode is a trademark of Linear Technology Corporation. Linear Technology Corporation 28 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear-tech.com losses. C4 provides a low impedance path to maintain an equal voltage swing in L1B, improving regulation flyback converter, during switch on time, all the converter’ ...