Key points for designing a universal multi-output monolithic voltage regulator

Multi-output monolithic regulators are easy to use and suitable for applications where multi-chip solutions are not feasible . However, the lack of options for input voltages above 30V and support for high output currents has limited the popularity of multi-output regulators. The LT3692A fills this gap with a dual-channel monolithic regulator that operates up to 36V input. In addition, the device includes a number of channel optimization features that make the LT3692A's per-channel performance comparable to multi-chip solutions.

The LT3692A is available in two packages: a 5mmx5mm QFN package and a 38-pin plastic TSSOP package. While both packages offer a full feature set, the TSSOP package enhances the thermal performance of the dual-channel buck regulator.

High input voltage and high transient withstand capability

The LT3692A operates from input voltages up to 36V and can withstand transient voltages up to 60V for 1 second, making it suitable for harsh operating environments, such as those commonly found in automotive environments.

On-chip temperature monitoring

The LT3692A provides an on-chip temperature monitoring function that simplifies application circuit design , debugging, and thermal optimization of the package. The voltage on the TJ pin is proportional to the Celsius temperature of the chip (i.e., 250mV is equivalent to 25°C, while 1.5V corresponds to 150°C).

The LT3692A TSSOP packaged die temperature was measured up to 80°C*, with both outputs supporting 3A loads (at 5V and 3.3V) at an 18V input voltage and a switching frequency of 400kHz. Figure 1 shows the schematic of the measured application circuit. However, for a 2.5A load, the maximum die temperature drops to 68°C with the same setup.

Standalone Comparator Components

The LT3692A also includes a stand-alone comparator block that provides a 720mV threshold with hysteresis and outputs an open collector signal. The comparator can be configured as a power good flag signal to monitor the output voltage by connecting the CMPI pin to the FB pin. This comparator can also be configured as a temperature flag that issues an alarm signal when the chip temperature rises to a preset point. This function is implemented in conjunction with the on-chip temperature monitor. Figure 2 shows how to configure a 100°C temperature flag.

Other Features

Independently Adjustable Current Limit

The switch current limit on each output can be set between 2A and 4.8A. This expands the number of load combinations that can be safely implemented without the risk of thermal overloading the package in extreme conditions such as short circuits. Likewise, current limiting can be used to protect the device in compact designs where the saturation margin of the inductor is reduced to meet size constraints.

Independent Sync

Independent synchronization allows any phase difference between the two outputs in addition to the standard 0° and 180°. The phase difference of the LT3692A is adjusted by controlling the duty cycle of the synchronization signal.

Frequency division

Frequency division enables the operating frequency of each channel to be adjusted to optimize overall performance and size. Channel 1 can be set to run at 1, 1/2, 1/4, or 1/8 of the frequency of Channel 2. Figure 3 shows the layout of an application circuit for a 3.3V/2.5A/550kHz channel and a 1.2V/1A/2.2MHz channel. The relatively low 550kHz frequency of VOUT1 boosts the input voltage of Channel 1 to 36V while meeting the minimum on-time requirement and maintaining high efficiency. The high 2.2MHz frequency of VOUT2 allows Channel 2 to use smaller components, as shown in Figure 3. Despite the reduced size, there is no degradation in electrical and thermal performance.

in conclusion

The LT3692A is a dual-channel output monolithic regulator that combines the ease of use and compact solution size of a typical monolithic regulator with the flexibility of a discrete multi-chip solution. Its high transient voltage withstand capability, chip temperature monitor, independent comparator building blocks, adjustable current limit, adjustable switching frequency and frequency division function, and independent synchronization features make the LT3692A suitable for many applications that other monolithic chips cannot do.