Datasheet LT8580 (Analog Devices) - 7

link to page 7 link to page 7 LT8580
operaTion
The LT8580 uses a constant-frequency, current mode con- an inverting configuration, the FBX pin is pulled down to trol scheme to provide excellent line and load regulation. 3mV by the RFBX resistor connected from VOUT to FBX. Refer to the Block Diagram for the following description Amplifier A1 becomes inactive and amplifier A2 performs of the part’s operation. At the start of each oscillator cycle, the noninverting amplification from FBX to VC. the SR latch (SR1) is set, which turns on the power switch, Q1. The switch current flows through the internal current
SEPIC Topology
sense resistor, generating a voltage proportional to the As shown in Figure 1, the LT8580 can be configured as switch current. This voltage (amplified by A4) is added a SEPIC (single-ended primary inductance converter). to a stabilizing ramp and the resulting sum is fed into the This topology allows for the input to be higher, equal, or positive terminal of the PWM comparator A3. When this lower than the desired output voltage. Output disconnect voltage exceeds the level at the negative input of A3, the is inherently built into the SEPIC topology, meaning no DC SR latch is reset, turning off the power switch. The level path exists between the input and output. This is useful at the negative input of A3 (VC pin) is set by the error for applications requiring the output to be disconnected amplifier A1 (or A2) and is simply an amplified version of from the input source when the circuit is in shutdown. the difference between the feedback voltage (FBX pin) and the reference voltage (1.204V or 3mV, depending on the
Inverting Topology
configuration). In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. The LT8580 can also work in a dual inductor inverting topology, as shown in Figure 2. The part’s unique feedback The LT8580 has an FBX pin architecture that can be used pin allows for the inverting topology to be built by simply for either noninverting or inverting configurations. When changing the connection of external components. This configured as a noninverting converter, the FBX pin is solution results in very low output voltage ripple due to pulled up to the internal bias voltage of 1.204V by the the inductor L2 in series with the output. Abrupt changes RFBX resistor connected from VOUT to FBX. Amplifier A2 in output capacitor current are eliminated because the becomes inactive and amplifier A1 performs the invert- output inductor delivers current to the output during both ing amplification from FBX to VC. When the LT8580 is in the off-time and the on-time of the LT8580 switch. V L1 C2 D1 C2 IN > VOUT L1 L2 OR • V • • OUT VIN VOUT VIN = VOUT OR VIN SW L2 VIN SW V D1 IN < VOUT + + C1 LT8580 • C1 LT8580 R1 R1 SHUTDOWN SHDN FBX SHUTDOWN SHDN FBX RT GND + RT GND C3 C3 VC VC + SYNC SS R SYNC SS C RC RT CSS RT CSS CC CC 8580 F01 8580 F02
Figure 1. SEPIC Topology Allows for the Input to Span Figure 2. Dual Inductor Inverting Topology Results in the Output Voltage. Coupled or Uncoupled Inductors Low Output Ripple. Coupled or Uncoupled Inductors Can Be Used. Follow Noted Phasing if Coupled Can Be Used. Follow Noted Phasing if Coupled
8580fa For more information www.linear.com/LT8580 7 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts