linear DESIGN FEATURES LTMag-V18N02-08-LT3570-ChrisFalvey service manual
Contents
1. 100 eee 40k 90 THA EFFICIENCY 80 a oe 70 7 F 60 x Ss e POWER LOSS 2 Z 50 A x 8 co i Ww g Viy 24V z 30 Vouts 12V a 20 2 1 MODE ae CONTINUOUS MODE 10 PULSE SKIPPING MODE H BURST MODE OPERATION 0 100 0 1 1 10 LOAD CURRENT A Figure 3 Efficiency for channel 3 in Figure 1 in each of the three modes of operation todrop current limit fold back protects the power components by reducing the current limit For predictable tracking current limit fold back is disabled during start up Input undervoltage lockout output overvoltage shutdown and thermal shutdown also protect the power components and the IC from damage Conclusion The LTC3853 s small footprint belies its versatility and extensive feature set From inputs up to 24V it can regulate three separate outputs or it can be configured for higher currents by tying channels 1 and 2 together Either way the phase relationship between channels is automatically optimized to reduce ripple currents At low duty cycles the short minimum on time ensures constant frequency operation and peak current limit remains constant even as duty cycle changes The cost effective LTC3853 incorporates these features and more into a 40 pin 6mm x 6mm QFN package 7 level of system integration it greatly simplifies board design for complex applic2. LI DESIGN FEATURES Buck Boost and LDO Regulators Combined in a 4mm x 4mm QFN Introduction The LT3570 simplifies complex multi rail power supply designs by integrating three DC DC regulators into a single package a current mode buck regulator a current mode boost regulator and an LDO controller The buck and boost regulators each have a current limit of 1 54 The LDO controller has an output current capability of 10mA and combines with an external NPN transistor to create a linear regulator The frequency of the switching regulators can be set from 500kHz to 3MHz by an external resistor or synchronized to an exter nal oscillator The independent input voltages for each regulator offers a wide operating range from 2 5V up to 40V Each regulator also has its own shutdown circuitry and the buck and boost regulators have their own soft start circuitry The typical application shown in Figure 1 generates 3 3V at 1A from the buck regulator 2 5V at 40mA from the LDO controller and 12V at 275mA 5V The LT3570 simplifies complex multi rail power supply designs by integrating three DC DC regulators into a single package a current mode buck regulator a current mode boost regulator and an LDO controller from the boost regulator all from a 5V input supply voltage and with an overall efficiency around 85 Features Available in either a 24 lead 4mm x 4mm QFN or a 20 pin TSSOP package the LT3570 is a constant fr
3. V INTV c is still available for ancillary keep alive circuits Pulse skipping mode is always enabled at start up to prevent sink ing current from a pre biased output voltage When the output reaches 80 of the set value the part switches over to forced continuous mode until the output has entered the POWER GOOD window at which point it switches over to the selected mode of opera tion Forced continuous mode reduces the output ripple as the power good threshold is crossed to ensure that the POWER GOOD indicators make just one low to high transition Three different max current com parator sense thresholds can be set via the ILIM pin The current is sensed using a high speed rail to rail differ ential current sense comparator The circuit of Figure 1 uses accurate sense resistors between the inductors and the outputs For reduced power loss at high load currents the LTC3853 can also monitor the parasitic resistance of the inductor DCR sensing Peak inductor current is limited on a cycle by cycle basis and is independent of duty cycle If load current is high enough to cause the feedback voltage to 3 3V at 500mA Figure 5 shows the load step response of Voyr and Vourz with a 200mA load step on Vouri Conclusion The LT3570 is a monolithic dual output switching regulator buck and boost with a NPN LDO controller and is ideal for a broad variety of applica tions Because the LT3570 offersa high DESIGN IDEAS LY
4. acitors Forced continuous mode offers fixed frequency operation from no load to full load providing the lowest output voltage ripple at the cost of light load efficiency Pulse skipping mode oper ates by preventing inductor current reversal by turning off the synchro nous switch as needed This mode is a compromise between the other two modes offering lower ripple than Burst Mode operation and better light load efficiency than forced continuous mode Regardless of the mode selected the LTC3853 operates in constant fre quency mode at higher load currents Figure 3 shows the efficiency in each of the three modes LT3570 continued from page 29 DSL Modem Figure 4 shows an application for a DSL modem or set top box The sup ply voltage for Viya comes from a wall adapter that can range from 8V to 30V This voltage is stepped down to 5V at 100mA for Voyrz which then supplies the power to drive both the boost regulator and LDO controller Voyr is set to 8V at 200mA and Voyr3 is set Linear Technology Magazine June 2008 Each of the LTC3853 s channels can be enabled with its own RUN pin or slewed up or down with its own TRACK SS pin Tracking holds the feedback voltage to the lesser of the internal reference voltage or the voltage on TRACK SS which can be brought up with an external ramp or with its own 1 2pA internal current source With all of the TRACK SS pins held low and any output enabled through its RUN pin the 5
5. ations that need multiple volt age supply rails With the flexibility of independent supply inputs and adjust able frequency the user can seta wide array of custom output voltages The LT3570 is a feature rich solution that satisfies the needs for multiple output voltages in a compact solution 7 41
6. ear Technology Magazine June 2008 DESIGN FEATURES LY Vint Vine Ving BIAS SHDN1 SHDN2 SHDN3 BOOST sw2 FB3 GND Figure 2 Dying gasp system keeps power even when battery is disconnected voltage during start up A controlled ramp reduces inrush current on the input supply and minimizes output overshoot An external capacitor con nected between the SS pin and ground programs the slew rate The voltage on the SS pin overrides the internal reference voltage to the error amplifier and is charged by a 4 5pA internal current source VIN 8V TO 30V BOOST C1 LT3570 NPN_DRV FB3 SYNC GND The BIAS pin allows the internal circuitry to draw current from a lower voltage supply than the input reduc ing power dissipation and increasing efficiency Normally the quiescent currentis supplied from Vno but when the voltage on the BIAS pin exceeds 2 5V the current is supplied from the BIAS pin The BIAS pinis only available on the 24 lead QFN package Figure 4 DSL modem application Linear Technology Magazine June 2008 Vout2 i 5V DIV i 1ms DIV Figure 3 Output waveforms when power is removed from the circuit in Figure 2 Applications Dying Gasp Application The LT3570 provides an ideal solu tion for any dying gasp system Figure 2 shows a typical application powering an airbag controller In an automobile accident the battery may get disconnected from the sh
7. equency current mode regulator If all SHDN pins are held low zero quiescent current is drawn from the input supplies and the part is turned Vint Vine Ving BIAS SHDN1 SHDN2 SHDN3 12V 275mA SYNC BOOST FB3 GND by Chris Falvey off Any SHDN pin voltage exceeding 1 5V will turn on the corresponding regulator A precise shutdown pin threshold allows for easy integration of input supply undervoltage lockout All three regulators share the same internal 800mV reference voltage For each regulator an external resistor divider programs the output voltage to any value above the part s reference voltage The switching frequency is set with an external resistor from the Rr pin to GND This allows a trade off between minimizing component size by using higher switching frequen cies and maximizing efficiency by using lower switching frequencies Additionally running at a low switch ing frequency allows for applications that require larger Vjy to Voyr ratios The adjustable and synchronizable switching frequency also allows the user to keep the switching noise out of critical wireless and audio bands Both the buck and boost regulators control the slew rate of the output s __ EFFICIENCY Vin 5V Voyt2 3 3V 75 Vout1 12V Ioyt1 275mA 4 VouT3 2 5V lout3 40mA 0 0 2 0 4 0 6 0 8 1 0 louta A Figure 1 A typical 5V input to 3 3V 2 5V and 12V application 28 Lin
8. ock sen sors yet the airbag must still fire In this application the battery supplies power to the boost regulator Voyry is set to 36V and drives Vz and Vms the inputs to the buck regulator and the LDO controller respectively Even after the input supply is removed the buck regulator and the LDO continue to function properly for more than 3ms as the energy continues to be supplied from the output capacitor of the boost regulator The buck regulator turns off when Vino approaches the input undervoltage lockout of 2 3V see Figure 3 continued on page 41 loutt 200mA DIV Vouti bas 100mV DIV VouT2 100mV DIV 100ys DIV Figure 5 Step response of Figure 4 with boost current stepped from 200mA to 400mA 29 lia 2A DIV ie ve 2A DIV VOUTI 2 ee SE a O 200mV DIV f 25us DIV Vin 14V VouT1 2 NOM 1 8V LOAD STEP ON Vout1 2 1A TO 6A Figure 2 Post package trimming of the LTC3853 s current sense comparators provides excellent current sharing between channels 1 and 2 even during a transient sync with frequencies between 250kHz and 750kHz The LTC3853 can be set to operate in one of three modes under light load conditions Burst Mode operation of fers the highest light load efficiency by switching in a burst of one to several pulses replenishing the charge stored in the output capacitors followed bya long sleep period when the load current is supplied by the output cap
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linear DESIGN FEATURES LTMag V18N02 08 LT3570 ChrisFalvey service manual