Datasheet LT8570, LT8570-1 (Analog Devices) - 8

link to page 8 link to page 8 LT8570/LT8570-1
OPERATION
The LT8570/LT8570-1 use a constant-frequency, current down to 3mV by the RFBX resistor connected from VOUT mode control scheme to provide excellent line and load to FBX. Amplifier A1 becomes inactive and amplifier A2 regulation. Refer to the Block Diagram for the following performs the noninverting amplification from FBX to VC. description of the part’s operation. At the start of each oscillator cycle, the SR latch (SR1) is set, which turns on
SEPIC Topology
the power switch, Q1. The switch current flows through As shown in Figure 1, the LT8570/LT8570-1 can be the internal current sense resistor, generating a voltage configured as a SEPIC (single-ended primary inductance proportional to the switch current. This voltage (amplified converter). This topology allows for the input to be higher, by A4) is added to a stabilizing ramp and the resulting sum equal, or lower than the desired output voltage. Output is fed into the positive terminal of the PWM comparator A3. disconnect is inherently built into the SEPIC topology, When this voltage exceeds the level at the negative input of meaning no DC path exists between the input and output. A3, the SR latch is reset, turning off the power switch. The This is useful for applications requiring the output to be level at the negative input of A3 (VC pin) is set by the error disconnected from the input source when the circuit is amplifier A1 (or A2) and is simply an amplified version of 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 LT8570/LT8570-1 can also work in a dual induc- tor inverting topology, as shown in Figure 2. The part’s The LT8570/LT8570-1 have an FBX pin architecture that can unique feedback pin allows for the inverting topology to be used for either noninverting or inverting configurations. be built by simply changing the connection of external When configured as a noninverting converter, the FBX pin components. This solution results in very low output is pulled up to the internal bias voltage of 1.204V by the voltage ripple due to the inductor L2 in series with the RFBX resistor connected from VOUT to FBX. Amplifier A2 output. Abrupt changes in output capacitor current are becomes inactive and amplifier A1 performs the inverting eliminated because the output inductor delivers current amplification from FBX to VC. When the LT8570/LT8570-1 to the output during both the off-time and the on-time of are in an inverting configuration, the FBX pin is pulled the LT8570/LT8570-1 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 LT8570/LT8570-1 • C1 LT8570/LT8570-1 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 8570 F01 8570 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
85701fa 8 For more information www.linear.com/LT8570 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts