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LT3471 Datasheet(PDF) 9 Page - Linear Technology |
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LT3471 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT3471 3471f APPLICATIONS INFORMATION Semiconductor MBRM120. This is a 20V diode. Where the switch voltage exceeds 20V, use the MBRM140, a 40V diode. These diodes are rated to handle an average for- ward current of 1.0A. In applications where the average forward current of the diode is less than 0.5A, use the Philips PMEG 2005, 3005, or 4005 (a 20V, 30V or 40V diode, respectively). LAYOUT HINTS The high speed operation of the LT3471 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 5 shows the recommended component placement. Compensation—Theory Like all other current mode switching regulators, the LT3471 needs to be compensated for stable and efficient operation. Two feedback loops are used in the LT3471: a fast current loop which does not require compensation, and a slower voltage loop which does. Standard Bode plot analysis can be used to understand and adjust the voltage feedback loop. As with any feedback loop, identifying the gain and phase contribution of the various elements in the loop is critical. Figure 6 shows the key equivalent elements of a boost converter. Because of the fast current control loop, the power stage of the IC, inductor and diode have been replaced by the equivalent transconductance amplifier gmp. gmp acts as a current source where the output current is proportional to the VC voltage. Note that the maximum output current of gmp is finite due to the current limit in the IC. From Figure 6, the DC gain, poles and zeroes can be calculated as follows: Figure 5. Suggested Layout Showing a Boost on SW1 and an Inverter on SW2. Note the Separate Ground Returns for All High Current Paths (Using a Multilayer Board) 10 GND GND SHDN/SS1 9 8 7 SHDN/SS2 FB1N R4 R2 R3 FB1P VOUT1 R1 C2 3471 F05 C3 CSS1 CSS2 RSS1 RSS2 C1 • • C4 D2 VOUT1 VOUT2 SW1 SW2 C5 D1 L1 L2 L3 VOUT2 FB2P FB2N VREF 6 1 2 3 4 5 LT3471 PIN 11 GND VCC GND GND GND CONTROL 1 CONTROL 2 Output Pole: P1= 2 2• • R Error Amp Pole: P2 = 1 2• • R Error Amp Zero: Z1= 1 2• • R DC GAIN: A = V V ESR Zero: RHP Zero: Z3 = High Frequency Pole: P3 > L O C REF OUT π π π = π π = • • • •• • • • •• • • •• • : • C C C gR g R Z RC VR VL f Phase Lead Zero Z OUT C C ma O mp L ESR OUT IN L OUT S 1 2 2 1 2 2 3 4 1 2 2 2 π π π •• : •• • • RC Phase Lead Pole P C RR RR PL PL 1 4 1 2 12 12 = + |
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