LT1376 Linear Technology Corporation, LT1376 Datasheet

Page 1

... A shutdown signal will reduce supply current both parts. The LT1375 can be externally syn- chronized from 550kHz to 1MHz with logic level inputs. The LT1375/LT1376 fit into standard 8-pin PDIP and SO packages, as well as a fused lead 16-pin SO with much lower thermal resistance. Full cycle-by-cycle short-cir- cuit protection and thermal shutdown are provided ...

Page 2

... Sense Voltage (Fixed 5V Part) .................................. 7V SYNC Pin Voltage ..................................................... 7V Operating Ambient Temperature Range LT1375C/LT1376C ................................. LT1375I/LT1376I ............................... – Operating Junction Temperature Range LT1375C/LT1376C ............................... 125 C LT1375I/LT1376I ............................. – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 C ...

Page 3

... 15V 1.5V, boost open, switch open CONDITIONS All Conditions All Conditions 5V V 25V 30V (LT1375HV/LT1376HV (Notes 2, 8) SHDN (Note 8) SHDN 1V 2. 4.4V SHDN FB ...

Page 4

... CONDITIONS V Open Device Shutting Down C V Open LT1375HV/LT1376HV Device Shutting Down C LT1375HV/LT1376HV Device Starting current is the current drawn by the BIAS pin when the bias pin is held at 5V. Total input referred supply current is calculated by summing input supply current ( TOT ...

Page 5

... ERROR AMPLIFIER EQUIVALENT CIRCUIT LOAD 500 –50 100 1k 10k 100k 1M 10M FREQUENCY (Hz) 1375/76 G03 LT1376 Minimum Input Voltage with 5V Output 8.5 MINIMUM INPUT VOLTAGE CAN BE REDUCED BY ADDING A SMALL EXTERNAL 8.0 PNP. SEE APPLICATIONS INFORMATION 7.5 MINIMUM 7.0 VOLTAGE TO START WITH 6.5 STANDARD CIRCUIT 6.0 MINIMUM VOLTAGE 5 ...

Page 6

... LT1375/LT1376 W U TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current 10V OUT 1. 10V OUT 1. 1.00 0. 0.50 0. INPUT VOLTAGE (V) 1375/76 G13 BOOST Pin Current 0.25 0.50 0.75 1.00 1.25 SWITCH CURRENT (A) 1375/76 G16 PIN FUNCTIONS ...

Page 7

... U U PIN FUNCTIONS BIAS (LT1376 Only): The BIAS pin is used to improve efficiency when operating at higher input voltages and light load current. Connecting this pin to the regulated output voltage forces most of the internal circuitry to draw its operating current from the output voltage rather than the input supply ...

Page 8

... U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1376 is used to set output voltage and also to provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin ...

Page 9

... Figure 2. Frequency and Current Limit Foldback equal to the short-circuit current limit of the switch (typi- cally 2A for the LT1376, folding back to less than 1A). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 500kHz, so fre- quency is reduced by about 5:1 when the feedback pin voltage drops below 1V (see Frequency Foldback graph) ...

Page 10

... Current rating decreases with duty cycle because the LT1376 has internal slope compensation to prevent cur- rent mode subharmonic switching. For more details, read Application Note 19. The LT1376 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit ...

Page 11

... Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1376 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage, and reduce core loss. Graphs in the Typical Performance Characteristics section show maximum output load current versus inductor size and input voltage ...

Page 12

... CORE physically smaller capacitors have high ESR. The ESR MATER- HEIGHT IAL (mm) range for typical LT1376 applications is 0. typical output capacitor is an AVX type TPS, 100 F at 10V, with a guaranteed ESR less than 0.1 . This is a “D” size KM 4.2 surface mount solid tantalum capacitor. TPS capacitors KM 4 ...

Page 13

... Design Note 95. OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT1376. Ripple voltage is determined by the high frequency impedance of the output capacitor, and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is: ...

Page 14

... SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1376. 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 15

... SWITCH NODE CONSIDERATIONS For maximum efficiency, switch rise and fall times are made as short as possible. To prevent radiation and high can be added to frequency resonance problems, proper layout of the com- FB ponents connected to the switch node is essential. B field LT1375/LT1376 OUTPUT V SW STANDBY + TOTAL SHUTDOWN ...

Page 16

... PC layout, you will see that it is irreducibly short. If you move the diode or input capacitor away from the LT1376, get your resumé in order. The other paths contain only some combination of DC and 500kHz triwave, so are much less critical. ...

Page 17

... The negative voltage can forward bias parasitic junctions on the IC chip and cause erratic switching. The LT1376 has special circuitry inside which mitigates this problem, but negative 5V/DIV 20ns/DIV Figure 7 ...

Page 18

... To calculate minimum operating input voltage, switch voltage loss and maximum duty cycle must be taken into account. With the LT1376, there is the additional consideration of proper operation of the boost circuit. The boost circuit allows the power switch to ...

Page 19

... FREQUENCY COMPENSATION Loop frequency compensation of switching regulators can be a rather complicated problem because the reactive components used to achieve high efficiency also LT1375/LT1376 V IN LT1375 SYNC 1375/76 F11 Figure 11. Gating the Sync Signal ...

Page 20

... The basic connections are shown in Figure 12. Figure 13 shows a Bode plot of the phase and gain of the power section of the LT1376, measured from the V pin to the output. Gain is set by the 2A/V transcon- ...

Page 21

... None of this will show on a theoretical Bode plot because Bode is an amplitude insensitive analysis. Tests have shown that if ripple voltage on the V LT1376 will be well behaved . The formula below will give an estimate of V loop, assuming that 500kHz ...

Page 22

... LT1375/LT1376 U U APPLICATIONS INFORMATION How Do I Test Loop Stability? The “standard” compensation for LT1376 is a 3.3nF capacitor for C , with While this compensation will C C work for most applications, the “optimum” value for loop compensation components depends, to various extent, on parameters which are not well controlled ...

Page 23

... THERMAL CALCULATIONS Power dissipation in the LT1376 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formu- las show how to calculate each of these losses. These ...

Page 24

... Example: with 0.5V 1.5A not take into account that maximum rated switch current ( the LT1376 is reduced slightly for duty cycles above 50%. If duty cycle is expected to exceed 50% (input 100 F 10V TANT voltage less than output voltage), use the actual I from the Electrical Characteristics table ...

Page 25

... This suggests a minimum inductor for this application, but looking at the ripple voltage chart shows that output ripple voltage could be reduced by a factor of two by using inductor. There is no rule LT1375/LT1376 ...

Page 26

... LT1375/LT1376 U U APPLICATIONS INFORMATION of thumb here to make a final decision. If modest ripple is needed and the larger inductor does the trick, go for it. If ripple is noncritical use the smaller inductor. If ripple is extremely critical, a second filter may have to be added in any case, and the lower value of inductance can be used. ...

Page 27

... U Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.130 0.005 0.045 – 0.065 (1.143 – 1.651) (3.302 0.127) 0.125 (3.175) MIN 0.018 0.003 (0.457 0.076) LT1375/LT1376 D2 1N914 OUTPUT 5V L1 C1** 100 F 10V TANT + C5** D3 100 F ...

Page 28

... LT1375/LT1376 PACKAGE DESCRIPTION 0.010 – 0.020 45 (0.254 – 0.508) 0.008 – 0.010 0 – 8 TYP (0.203 – 0.254) 0.016 – 0.050 0.406 – 1.270 * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" ...