ANALOG DEVICES LTC3311-0.875 2MHz Automotive Low EMI Buck Regulator Instruction Manual

ANALOG DEVICES LTC3311-0.875 2MHz Automotive Low EMI Buck Regulator Instruction Manual

DESCRIPTION

Demonstration circuit 3127A features the LTC®3311-0.875, fixed 0.875V, 12.5A synchronous step-down Silent
Switcher® operating as a 2MHz, 3V to 3.6V input, 0.875V/12.5A output buck regulator. The LTC3311-0.875
supports fixed 0.875V output voltage with operating frequencies from 500kHz up to 5MHz. The LTC3311-0.875
is a compact, ultralow emission, high efficiency, and high speed synchronous monolithic step-down switching reg-
ulator. The LTC3311-0.875 has Active Voltage Positioning (AVP) where the output voltage is dependent on load current. At light loads the output voltage is regulated above the nominal value. At full load the output voltage is regulated below the nominal value. The DC load regulation is adjusted to improve transient performance and reduce required output capacitance.

DC3127A is set up to run in forced continuous mode with a 2MHz switching frequency but can be configured
to run at different switching frequencies, or in pulse skip mode. The LTC3311-0.875 oscillator can also synchro-
nize to an external clock using MODE/SYNC turret, with the DC3127 default setup. Figure 5 shows the efficiency
and power loss of the DC3127A with a 3.3V input in both operation modes.

The DC3127A is optimized for keeping its output within ±3% of the target voltage during a 4A to 8A, 4A/µs load
step transient. A fast load step transient circuit is placed on the back of the demo board to measure the load step response of the converter.

The DC3127A also has an EMI filter to reduce conducted EMI. This EMI filter can be included by applying the input
voltage at the VIN EMI terminal. The EMI performance of the board is shown in the EMI TEST RESULTS section.
The red lines in the EMI performance graphs illustrate the CISPR25 Class 5 peak limits for the conducted and
radiated emission tests.

The LTC3311-0.875 data sheet gives a complete description of the part and its application information. The data
sheet must be read in conjunction with this demo manual.
The LTC3311-0.875 is assembled in a 3mm × 3mm LQFN package with exposed pads for low thermal resistance.
The layout recommendations for low EMI operation and maximum thermal performance are available in the data sheet section Low EMI PCB Layout.

Design files for this circuit board are available.

All registered trademarks and trademarks are the property of their respective owners.

PERFORMANCE SUMMARY

Specifications are at TA = 25°C

BOARD PHOTO

Figure 1. DC3127A Board Photo

QUICK START PROCEDURE

Refer to Figure 2 for the proper measurement equipment setup and follow the procedure below:
NOTE: For accurate VIN, VOUT and efficiency measurements, measure VIN at the VIN SNSE and GND SNSN tur-
rets, and measure VOUT at the VOUT SNSE and GND SNSE turrets as illustrated as VM1 and VM2 in Figure 2. When measuring the input or output ripple, care must be taken to avoid a long ground lead on the oscilloscope probe.

1. Set the JP1 Jumper to the HI position.

2. With power off, connect the input power supply to VIN and GND. If the input EMI filter is desired, connect the input power supply to VIN EMI and GND.

3. Set power supply PS1 current limit to 10A. Set the electronic load LD1 to CC mode and 0A current. Slowly increase PS1 to 1V. If PS1 output current reads less than 20mA, increase PS1 to 3.3V. Verify that VM1 reads 3.3V and VM2 reads around 0.89V. Check VM1, VM2, VM3, PS1 output current and LD1 input current. Connect an oscilloscope voltage probe as shown in Figure 3. Set Channel to AC-coupled, voltage scale to 20mV and time base to 10µs. Check VOUT ripple voltage. Verify that PGOOD voltage is above 3V.

4. Increase the load by 1A intervals up to 12.5A and observe the voltage output regulation, ripple voltage, and the voltage on the SSTT turret. Calculate Die temperature using the formula below:

5. If pulse skipping mode is desired, set PS1 to 0V. In- stall a 0Ω resistor in the R5 location and remove R3.
   Repeat steps 1 through 4. In step 4, observe that the switching waveform is now in pulse skipping mode at light load.

6. Optional: To change the frequency, remove R4 and R5, if installed. Install the desired RT resistor in the R6 location. Note that the MODE/SYNC pin should have high impedance to GND and VIN. Size the inductor, output capacitors and compensation components to provide the desired inductor ripple and a stable output. Refer to the LTC3311-0.875 data sheet and LTPowerCAD® for more information on choosing the required components.

7. To test the transient response with a base load, add the desired resistor to produce a minimum load between
   VOUT and I_STEP turrets (RL shown on Figure 2).
   Note that the total load resistance will be RL plus R10 (20mΩ). Adjust a signal generator with a 10ms period, 10% duty cycle and an amplitude from 1V to 2V to start.

8. Measure the I_STEP voltage to observe the current,VI_STEP /20mΩ. Adjust the amplitude of the pulse to
   provide the desired transient. Connect signal generator SG1 between SG_INPUT and GND turrets. Adjust the
   rising and falling edge of the pulse to provide the desired ramp rate. Figure 7 shows a load step from 4A to 8A. Refer to the following equations:

9. When done, turn off SG1, PS1 and Load. Remove all the connections to the demo board.

TEST SETUP

Figure 2. Test Setup for DC3127A Demo Board

Figure 3. Technique for Measuring Output Ripple and Step Response

TYPICAL PERFORMANCE CHARACTERISTICS

Figure 4. DC3127A, 0.875V/12.5A Step-Down Converter Typical Solution Schematic

Figure 5. Start-Up Waveforms with Light Load

Figure 6. Efficiency vs Load Current

Figure 7. Load Step Response

EMI TEST RESULTS

Figure 8. DC3127A CISPR25 Conducted Emission with Class 5 Peak Limits (Voltage Method)

Figure 9. DC3127 CISPR25 Radiated Emission (Horizonal)

Figure 10. DC3127 CISPR25 Radiated Emission (Vertical)

THEORY OF OPERATION

Introduction to the DC3127A

The DC3127A demonstration circuit features the LTC33110.875, a low voltage synchronous step-down Silent
Switcher. The LTC3311-0.875 is a monolithic, constant frequency, current mode step-down DC/DC converter. An
oscillator turns on the internal top power switch at the beginning of each clock cycle. Current in the inductor then
increases until the top switch comparator trips and turns off the top power switch. The peak inductor current, at
which the top switch turns off, is controlled by the voltage on the ITH node. The error amplifier servos the ITH node by comparing the voltage on the VFB pin with an internal 500mV reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference, leading the error amplifier to raise the ITH voltage until the average inductor current matches the new load current. When the top switch turns off, the synchronous bottom power switch turns on until the next clock cycle begins. In pulse skip mode, the bottom switch also turns off when inductor current falls to zero. If overload conditions result in excessive current flowing through the bottom switch, the next clock cycle will be delayed until the switch current returns to a safe level.

If the EN pin is low, the LTC3311-0.875 is in shutdown state with low quiescent current. When the EN pin is above its threshold, the switching regulator will be enabled.

The MODE/SYNC pin synchronizes the switching frequency to an external clock. It can be a clock output for multi-phase operation. It also sets the regulator operation modes. The operation modes are either forced continuous
or pulse skipping. See the LTC3311-0.875 data sheet for more detailed information.

The maximum allowable operating frequency is influenced by the minimum on time of the top switch, the ratio of VOUT to VIN and the inductance of the output inductor. The maximum allowable operating frequency may be calculated in the formula below.

Select an operating switching frequency below fSW(MAX).
Typically, it is desired to obtain an inductor current of 30% of the maximum LTC3311-0.875 operating load, 12.5A.
Use the formulas below to calculate the inductor value to obtain a 30% (3A) inductor ripple for the operating frequency.

When determining the compensation components, C10, C11 and R8, controlling the loop stability and transient
response are the two main considerations. The LTC33110.875 has been designed to operate at a high bandwidth
for fast transient response capabilities. This reduces required output capacitance to meet the desired transient
voltage range. The mid-band gain of the loop increases with R8 and the bandwidth of the loop increases with decreasing C11. C10 along with R8 provides a high frequency pole to reduce the high frequency gain.

Loop stability is generally measured using the Bode Plot method of plotting loop gain in dB and phase shift in
degrees. The 0dB crossover frequency should be less the 1/6 of the operating frequency to reduce the effects of added phase shift of the modulator. The control loop phase margin goal should be 45° or greater and a gain margin goal of 8dB or greater.

PARTS LIST

SCHEMATIC DIAGRAM

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ESD Caution
ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.

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