LT1959 LINER [Linear Technology], LT1959 Datasheet
LT1959
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LT1959 Summary of contents
Page 1
... Efficiency is maintained over a wide output current range by keeping quiescent supply current to 4mA and by utiliz- ing a supply boost capacitor to saturate the power switch. The LT1959 fits into standard 7-pin DD and fused lead SO-8 packages. Full cycle-by-cycle short-circuit protection and thermal shutdown are provided. Standard surface mount external parts are used, including the inductor and capacitors ...
Page 2
... Pin High Clamp C Switch Current Limit Slope Compensation 2 U RATINGS (Note 1) SYNC Pin Voltage ..................................................... 7V Operating Junction Temperature Range LT1959C ................................................ 125 C LT1959I ........................................... – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 ORDER PART NUMBER V IN LT1959CR BOOST ...
Page 3
... Note 6: Input supply current is the bias current drawn by the input pin with switching disabled. Note 7: Switch on resistance is calculated by dividing V by the forced current (4.5A). See Typical Performance Characteristics for the graph of switch voltage at other currents. LT1959 = 1.5V, Boost = V + 5V, switch open, unless C IN ...
Page 4
... LT1959 W U TYPICAL PERFORMANCE CHARACTERISTICS Minimum Input Voltage with 3.3V Output 4.7 4.5 4.3 4.1 3.9 3.7 3.5 3 100 1000 LOAD CURRENT (mA) 1959 G01 Shutdown Pin Bias Current –500 AT 0.37V SHUTDOWN THRESHOLD. AFTER SHUTDOWN, CURRENT – 400 DROPS TO A FEW A – 300 – 200 – 2.38V LOCKOUT THRESHOLD – ...
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... SWITCH CURRENT (A) 1959 G15 Switch Voltage Drop 500 450 400 350 300 250 200 150 100 125 100 0 1959 G17 LT1959 Maximum Load Current OUT 4 4 4.0 3.8 3 3.4 3.2 3.0 L= 1.8 H 2.8 2.6 125 INPUT VOLTAGE (V) Current Limit Foldback ...
Page 6
... LT1959 PIN FUNCTIONS FB: The feedback pin is used to set output voltage using an external voltage divider that generates 1.21V at the pin with the desired output voltage. Three additional functions are performed by the FB pin. When the pin voltage drops below 0.8V, switch current limit is reduced. Below 0.7V the external sync function is disabled and switching fre- quency is reduced ...
Page 7
... W BLOCK DIAGRAM The LT1959 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 amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis ...
Page 8
... U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1959 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 voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin ...
Page 9
... Current rating decreases with duty cycle because the LT1959 has internal slope compensation to prevent cur- rent mode subharmonic switching. For more details, read Application Note 19. The LT1959 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit ...
Page 10
... Choose a value in microhenries from the graphs of maximum load current and core loss. Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1959 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed core geometries like toroids ...
Page 11
... ESR. The ESR Kool registered trademark of Magnetics, Inc range for typical LT1959 applications is 0. typical output capacitor is an AVX type TPS, 100 F at 10V, CORE MATER- HEIGHT with a guaranteed ESR less than 0.1 . This is a “D” size ...
Page 12
... OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT1959. 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 13
... SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1959. Typically, ULVO 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 14
... V = Hysteresis in input voltage level Example: output voltage is 5V, switching is to stop if input voltage drops below 6V and should not restart unless input rises back to 7.5V therefore 1.5V and V Let R = 25k LT1959 IN 2.38V 3.5 A SHDN 0.4V GND Figure 4. Undervoltage Lockout can be ...
Page 15
... PC layout, you will see that it is irreducibly short. If you move the diode or input capacitor away from the LT1959, get your resumé in order. The other paths contain only some combination of DC and 500kHz triwave, so are much less critical. ...
Page 16
... When looking at this, a >100MHz oscilloscope must be used, and waveforms should be observed on the leads of the package. This switch off spike will also cause the SW node to go below ground. The LT1959 has special circuitry inside which 5V/DIV 20ns/DIV Figure 7. Switch Node Resonance ...
Page 17
... It is common practice therefore to simply use the worst-case value and assume that RMS ripple current is one half of load current. At maximum output current of 4.5A for the LT1959, the input bypass capacitor should be rated at 2.25A ripple current. Note however, that there are many secondary considerations in choosing the final ripple current rating ...
Page 18
... IN ated by the inductor. The basic connections are shown in Figure 9. Figure 10 shows a Bode plot of the phase and gain of the power section of the LT1959, measured from the V pin to the output. Gain is set by the 5.3A/V transconduc- = 3A: OUT tance of the LT1959 power section and the effective complex impedance from output to ground ...
Page 19
... First, the combina- tion of output capacitor ESR and a large value for R cause loop gain to stop rolling off altogether, creating a gain margin problem. An approximate formula for R where gain margin falls to zero is: R Loop C LT1959 3000 200 PHASE 2500 150 GAIN ...
Page 20
... Bode plot because Bode is an amplitude insensitive analysis. Tests have shown that if ripple voltage on the V is held to less than 100mV C LT1959 will be well behaved . The formula below will give an estimate of V ripple voltage when R C loop, assuming that R ...
Page 21
... Note that fre- quency of the light load ringing may vary with component tolerance but phase margin generally hangs in there. CURRENT SHARING MULTIPHASE SUPPLY . Do this only The circuit in Figure 15 uses multiple LT1959s to produce C a 2.5V, 12A power supply. There are several advantages to above 1k C pin ripple ...
Page 22
... V to switch current transcon- C ductance prevents current hogging. A common V voltage forces each LT1959 to operate at the C same switch current, not duty cycle. Each device operates at the duty cycle defined by its respective input voltage. In Figure 15, the input could be split and each device oper- ated at a different voltage ...
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... BH ELECTRONICS #501-0726 1959 F16 ** TOKIN IE475ZY5U-C304 R4 † IF LOAD CAN GO TO ZERO, AN OPTIONAL 47k PRELOAD MAY BE USED TO IMPROVE LOAD REGULATION D1, D3: MBRD340 LT1959 D2 1N914 C2 0.27 F L1* BOOST 6 LT1959 SHDN GND V 7.87k 25V D1 1.5nF 2.49k CERAMIC + + C5** ...
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... LT1959 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 45 (0.254 – 0.508) 0.008 – 0.010 0 – 8 TYP (0.203 – 0.254) 0.016 – 0.050 (0.406 – ...