Automotive Battery Power Solution Based on Triple Output Regulator

DC/DC converters for automotive applications must operate in extreme environments where input transients can exceed the nominal battery voltage by 5 times and last for hundreds of milliseconds, while under-the-hood temperatures can rise dramatically to levels far beyond what typical commercial-grade ICs can withstand. In this harsh environment, space is at a premium, so even the most powerful devices must perform multiple functions.

The LT3694/LT3694-1 combines a 2.6A switching regulator and two low dropout linear regulators in a compact 4mm x 5mm QFN or thermally enhanced TSSOP package to meet these demanding requirements. The switching regulator requires only a single inductor and features an internal power switch, cycle-by-cycle current limit, and tracking/soft-start control. Each LDO requires only an external NPN pass transistor and features foldback current limit and tracking/soft-start control. An internal overvoltage detector shuts down the switching regulator when VIN exceeds 38V, protecting the switch and Schottky rectifier. This allows the device's VIN pin to withstand transients up to 70V without damaging the device itself or the rectifier.

4V to 36V Input Switching Regulator

The LT3694/LT3694-1 consists of a 36V monolithic switching regulator that can deliver up to 2.6A of output current from an input voltage as low as 4V. The output voltage can be set as low as a 0.75V feedback reference.

The regulator uses a current mode, constant frequency architecture that keeps loop compensation simple. External compensation allows customization of loop bandwidth, transient response, and phase margin.

Two low dropout linear regulators

The LT3694/LT3694-1 include two LDO linear regulators that use an external NPN pass transistor to provide up to 0.5A of output current. The base driver can provide up to 10mA of base current to the pass transistor and is current limited. The LDOs are internally compensated and stable with 2.2μF or greater output capacitance. The LDOs use the same 0.75V reference as the switching regulators.

If the BIAS pin is at least 0.9V higher than the DRIVE pin voltage, the LDO draws drive current from the BIAS pin, otherwise the LDO draws drive current from VIN. This reduces the power dissipation of the LDO, especially when VIN is relatively high.

The LDO implements foldback current limiting by monitoring the sense resistor on the collector of the NPN pass transistor. The initial threshold is set at 60mV, but folds back as VFB decreases until VFB = 0 and the threshold is 26mV. The 0.1Ω sense resistor sets the operating current limit to 600mA, but the short-circuit current limit drops to 260mA. This reduces the power dissipation in the pass transistor when the output is shorted.

Figure 1: LT3694/LT3694-1 in a wide input range, 3 output application

The LDOs can be shut down by pulling the FB pin above 1.25V with at least 30μA. If independent control of the LDOs is desired, the output of each LDO can be forced to 0V by pulling its TRK/SS pin low. If tracking or soft-start functionality is desired, use an open-drain output in parallel with the tracking or soft-start circuit described below. If tracking and soft-start are not required, a standard CMOS output (1.8V to 5V) with a 1kΩ series resistor works well.

Tracking/soft-start control

The buck regulator and each LDO have their own tracking/soft-start (TRK/SS) pin. When this pin is below the 0.75V reference, the regulator forces its feedback pin to be equal to the TRK/SS pin voltage instead of the reference voltage. The TRK/SS pin has a 3μA pull-up current source.

The soft-start function requires a capacitor from the TRK/SS pin to ground. At startup, this capacitor is at 0V, which drops the regulator output to 0V. A current source slowly charges the capacitor, causing its voltage to rise while the regulator output ramps up proportionally. Once the capacitor voltage reaches 0.75V, the regulator locks to the internal reference instead of the TRK/SS voltage. As soon as any shutdown event (overvoltage, overtemperature, undervoltage) occurs, the TRK/SS pin is pulled low to discharge the soft-start capacitor.

Figure 2: Ratio tracking waveform

Tracking is accomplished by connecting the TRK/SS pins of the slave regulators to a resistor divider from the output of the master regulator. The master regulator uses a common soft-start capacitor as described above to generate the startup ramp that controls the other regulators. The resistor divider ratio sets the type of tracking, either coincident tracking (ratio equal to the ratio of the slave feedback divider) or ratiometric tracking (ratio equal to the ratio of the master feedback divider plus a small offset). The TRK/SS pins can also be connected together to a single capacitor to provide ratiometric tracking, but this can only be done if the LDO is not shut down by pulling the FB pin high (see the "Two Low Dropout Linear Regulators" section above).

Figure 3: Switching regulator efficiency

Enable and undervoltage protection

The LT3694/LT3694-1 provides an enable and user-programmable undervoltage lockout function using the EN/UVLO pin. The undervoltage lockout protects against pulse stretching. This feature also protects the input source from excessive current because the buck regulator is a constant power load and draws more current when the input source is low. The undervoltage lockout shuts down all three regulators when a transition occurs.

Both functions use a pair of built-in comparators at the EN/UVLO input. The enable comparator has a 0.5V threshold and activates the bias circuitry inside the LT3694/LT3694-1. When EN/UVLO is below the enable threshold, the LT3694/LT3694-1 is in shutdown, drawing less than 1μA at 12V input. The undervoltage comparator has a 1.2V threshold and has 2μA of hysteresis. The UVLO hysteresis is a current sink that activates when EN/UVLO falls below the 1.2V threshold. A resistor divider from VIN to the EN/UVLO input sets the trip voltage and hysteresis. The undervoltage threshold maintains good accuracy over temperature to achieve tight control of the trip voltage. If this function is not used, the EN/UVLO pin should be tied to VIN.

Frequency Control

The switching frequency is adjustable from 250kHz to 2.5MHz and is set with a single resistor connected to the RT pin. Higher frequencies allow the use of smaller inductors and capacitors, but consume more power and result in a smaller allowable step-down range due to minimum on- and off-time restrictions.

Here we can see the difference between the LT3694 and the LT3694-1. The LT3694’s switching frequency can be synchronized to an external clock connected to the SYNC pin . The resistor on the RT pin should be set to provide a free-running frequency that is 20% lower than the synchronization frequency. The LT3694-1 replaces the SYNC pin with a CLKOUT pin, allowing the LT3694-1 to be used as a master clock to synchronize other switching regulators. CLKOUT generates a clock signal that runs at approximately 50% duty cycle.

3-Output Converter with Voltage Tracking

Figure 1 shows a converter with a wide input range of 6.3V to 36V, which generates three outputs: 5V, 3.3V, and 2.5V. These outputs can achieve ratiometric tracking by setting a common TRK/SS. Figure 2 shows the startup waveforms and enable signals of the three outputs. Figure 3 shows the efficiency of the switching regulator at different input voltages.

in conclusion

The LT3694/LT3694-1 includes three regulators in a tiny 4mm x 5mm QFN or 0-lead TSSOP package, providing a rugged and compact power solution. One regulator is a high efficiency switching regulator, and the other two are low noise, low dropout linear regulators. Only a few small external components are required to build an extremely compact 3-output solution.