Design Note 382: Low EMI Synchronous DC/DC Step-Down Controllers Offer Programmable Output Tracking

Introduction

The LTC3808 synchronous DC/DC step-down controller packs numerous features required by the latest electronic devices into a low profile (0.75mm) 3mm × 4mm leadless DFN package or a leaded SSOP-16 package. Two similar parts, the LTC3809 and LTC3809-1, are even smaller, but less feature-rich versions of the LTC3808. The LTC3809 family is available in a 3mm × 3mm leadless DFN package or a 10-pin MSOP Exposed Pad package. All three parts can provide output voltages as low as 0.6V and output currents as high as 7A from a 2.75V to 9.8V input range, making them ideal devices for one or two lithium-ion cell inputs as well as distributed DC power systems.

The LTC3808 and LTC3809 also include important features for noise-sensitive applications, including a phase-locked loop (PLL) for frequency synchronization and spread spectrum frequency modulation to minimize generated electromagnetic interference (EMI). The adjustable operating frequency (300kHz to 750kHz) allows the use of small surface mount inductors and ceramic capacitors for compact power supply solutions.

Other features include:

Low operating quiescent current to improve battery life and light load efficiency
No R_SENSE^™ current mode technology which senses the voltage across the main (top) power MOSFET to improve efficiency and reduce the size and cost of the solution
Current mode control for excellent AC and DC line and load regulation
Low dropout (100% duty cycle) for maximum energy extraction from a battery source
Output overvoltage protection and short circuit current limit protection
Adjustable or fixed built-in soft-start timer
Output voltage ramp control and the ability to track other voltage sources (LTC3808 and LTC3809-1)
PowerGood voltage monitor (LTC3808)

Table 1 compares the features of these three parts.

Table 1.
	START-UP CONTROL	SPREAD SPECTRUM	ADJUSTABLE FREQ/PLL	POWER GOOD
LTC3808	INTERNAL EXTERNAL TRACKING	Yes	Yes	Yes
LTC3809	INTERNAL	Yes	Yes	No
LTC3809-1	INTERNAL EXTERNAL TRACKING	No	No	No

Three Choices for Start-Up Control

The start-up of V_OUT for the LTC3808 and LTC3809-1 is based on the three different connections to the TRACK/SS pin. A typical application is shown in Figure 2. When TRACK/SS is connected to V_IN, the start-up of V_OUT is controlled by the internal soft-start which ramps from 0V to (V_FB) in about 1ms. A second start up mode allows the 1ms soft-start time to increase or decrease by connecting an external capacitor C_SS between the TRACK/SS pin and ground. An internal 1μA current source and the value of C_SS control the ramp time of TRACK/SS from 0V to above 0.6V. In this case, the LTC3808 and LTC3809-1 regulate the VFB to the voltage at the TRACK/SS pin instead of the internal soft-start ramp. The third mode allows V_OUT of the LTC3808 and LTC3809-1 to track an external voltage, V_X, during start-up if a resistor divider from V_X is connected to the TRACK/SS pin. Figure 3 shows the start-up of V_OUT in these tracking modes for the circuit shown in Figure 2.

For simplicity, the LTC3809 only offers a 1ms internal soft-start.

Low EMI DC/DC Conversion

The LTC3808 and LTC3809 minimize the need for EMI shields and filters in applications such as navigation systems, wireless LANs, data acquisition boards and industrial and military radio devices by optionally spreading the nominal operating frequency (550kHz) over a range of frequencies between 460kHz and 635kHz. Spread spectrum frequency modulation is enabled by biasing the SYNC/MODE pin to a DC voltage between 1.35V and (V_IN – 0.5V). An internal 2.6μA pull-down current source at the SYNC/MODE pin can be used to set the DC voltage at this pin by tying a resistor with an appropriate value between SYNC/MODE and V_IN. Figure 1 shows the application circuit and Figure 4 shows the frequency spectral plots of the output (V_OUT) with and without spread spectrum modulation. Note the significant reduction in peak output noise (>20dBm) with spread spectrum enabled.

Figure 1. Synchronous Converter with Spread Spectrum Frequency Modulation.

Figure 2. The LTC3808 Offers the Flexibility of Start-Up Control Based on the Three Different Connections on the TRACK/SS Pin.

Figure 3. Start-Up Output Voltage Tracking Plots for Circuit in Figure 2.

Figure 4. Comparison of the VOUT Spectrum with and without Spread Spectrum Modulation Enabled.

Figure 4. Comparison of the V_OUT Spectrum with and without Spread Spectrum Modulation Enabled.

Conclusion

The LTC3808, LTC3809 and LTC3809-1 offer flexibility, high efficiency, low EMI and many other popular features in small thermally efficient packages. They offer excellent solutions for low voltage portable and distributed power systems that require a small footprint, high efficiency and low noise.