LT3078

• Input voltage range: 0.6 V to 5.5 V
• BIAS voltage range: 2.375 V to 5.5 V
• Jumpers program output voltage according to selection matrix: 0.5 V to 4.2 V
• Maximum output current: 5 A
• BNC connectors for noise and PSRR measurement
• Jumper and resistor combinations select either 5.5 A or 2.5 A output current limit and commensurate monitoring or disable programmed current limit and monitoring
• Jumper turns regulator on or off
• Terminals provide output current, temperature, and output regulation status monitoring
• Jumper to margin output voltage ±2.5%
• The VIOC pin of the LT3078 manages power dissipation and PSRR
• Banana jacks minimize V_IN and V_OUT connection voltage drops
• VO+, VO−, and VI+ terminals for regulation and dropout monitoring
• Configurable load-transient circuit for load response testing
• Thermally enhanced, 22-lead, 3 mm × 4 mm x 0.95 mm, LQFN package

The EVAL-LT3078-AZ evaluation board features the LT3078, a 5 A, ultra-low noise, high power-supply rejection ratio (PSRR), 55 mV dropout ultra-fast linear regulator. The input voltage (V_IN) range for the V_IN power is from 0.6 V to 5.5 V. There are jumpers to set a 3-bit trilevel code that determines the output voltage (V_OUT) at preprogrammed levels that range from 0.5 V to 4.2 V. The maximum output current is 5 A. The EVAL-LT3078-AZ requires an external BIAS voltage (VBIAS) that is at least 1.2 V higher than V_OUT and is between 2.375 V and 5.5 V.

The LT3078 of the EVAL-LT3078-AZ requires few external components, therefore, simplifying circuit design. External component choice with careful printed circuit board (PCB) design helps to optimize noise, PSRR, load-transient response, and V_OUT regulation performance. The LT3078 requires capacitors for the internal reference, power input, BIASF pin, and the power output. The internal reference is bypassed with a 16 V, 0805 sized, 4.7 μF capacitor to reduce output noise and program the soft-start. Larger capacitor case sizes and higher voltage ratings decrease 1/f noise for otherwise comparable capacitors. The 10 μF x2 capacitor at the circuit output was chosen for high-frequency PSRR performance and to optimize V_OUT response during load transients.

The capacitor that bypasses the V_IN power for the LT3078 and the corresponding V_IN PCB layout can affect PSRR (for additional information, see the Best PSRR Performance: PCB Layout for Input Traces section). The EVAL-LT3078-AZ decouples the V_IN power with a 47 μF capacitor. Less V_IN capacitance can improve PSRR at high frequencies (for the minimum capacitor value required for V_IN, see the LT3078 data sheet). Note that a bulk 220 μF tantalum polymer capacitor further reduces V_IN variation during load transients and reduces input voltage ringing that can be caused by inductive input power leads. The PCB has a footprint for an optional Subminiature Version A (SMA) connector that allows a shielded V_IN power connection to the PCB edge, if required.

The EVAL-LT3078-AZ bypasses the BIASF pin with a 2.2 μF capacitor instead of the VBIAS supply input. Because the BIASF pin is isolated from VBIAS by a resistance that is internal to the LT3078, there is less PSRR degradation when BIASF is bypassed compared to when VBIAS is bypassed. Otherwise, the effect on PSRR of the V_IN and VBIAS bypass capacitors is similar.

The EVAL-LT3078-AZ has resistors that allow a CURRENT LIMIT jumper to select output current limits of either 2.5 A or 5.5 A. The CURRENT LIMIT jumper can also disable external current-limit programming by shorting the IMON pin to ground. An IMON terminal is available for current monitoring. The IMON voltage is the product of the resistance that externally programs current limit and the IMON pin current that is 1/5000 of the output current. Externally programmed current limit occurs when the IMON voltage is 1 V.

A POWER jumper (JP1) is available on the EVAL-LT3078-AZ to either connect the EN pin to VBIAS to turn the output on or to ground to disable the output. A TEMP terminal is also available for die temperature monitoring. There is a PG terminal that is pulled up to VBIAS by a 51 kΩ resistor and pulled down by the open-drain, negative channel metal-oxide semiconductor (NMOS) PG pin output for indication of regulator output status and other fault modes. The voltage input-to-output control (VIOC) terminal allows connections for automatically controlling a preregulation voltage. In addition, a MARG jumper can margin the output voltage to either ±2.5%.

Banana jacks minimize voltage drops on V_IN and V_OUT connections. Bayonet Neill-Concelman (BNC) connectors provide low noise connections to power V_IN, VBIAS, and V_OUT. The EVALLT3078- AZ PCB design uses a split-capacitor technique to Kelvin connect the ground terminal of the REF capacitor to the ground terminal of the output capacitor, and the SENSE pin to the positive terminal of the output capacitor. The VO+, VO−, and VI+ terminals Kelvin connect to V_IN and V_OUT and are the optimum place to observe output voltage regulation and dropout voltage performance. There are test points for BIASF and REF voltages.

The EVAL-LT3078-AZ includes a load-transient circuit for load response testing. A 2512 size power resistor must be installed with a resistance that achieves the required transient current, but all the remaining circuit components are provided including a 10 kΩ gate pull-down resistor, a 4.7 nF Miller capacitor and a high-current NMOS. Additionally, a banana jack makes it possible to return an external DC load through the same 0.02 Ω sense resistor that is the return for the transient current so both can be monitored together.

The EVAL-LT3078-AZ has placeholders identified on the schematic as optional DNI components that make it convenient to add capacitance (for more information, see Figure 8 in the user guide).

Full specifications on the LT3078 are available in the LT3078 data sheet available from Analog Devices, Inc., and must be consulted with this user guide when using the EVAL-LT3078-AZ evaluation board. The LT3078 of the EVAL-LT3078-AZ features a thermally enhanced, 22-lead, 3 mm x 4 mm x 0.95 mm LQFN package. Proper board layout is essential for maximum thermal performance.