36V, 3.5A Dual Monolithic Buck with Integrated Die Temperature Monitor and Standalone Comparator Block Design Note 492 Edwin Li INTRODUCTION HIGH INPUT VOLTAGE WITH HIGH TRANSIENT CAPABILITY Multioutput monolithic regulators are easy to use and fit into spaces where multichip solutions cannot. Nevertheless, the popularity of multioutput regulators is tempered by a lack of options for input voltages above 30V and support of high output currents. The LT3692A fills this gap with a dual monolithic regulator that operates from inputs up to 36V. It also includes a number of channel optimization features that allow the LT3692A’s per-channel performance to rival that of multichip solutions. The LT3692A can operate up to an input voltage of 36V and can sustain a transient voltage up to 60V for 1 second, making it suitable for harsh operating environments such as those commonly found in automotive environments. ON-DIE TEMPERATURE MONITORING The LT3692A provides an on-die temperature monitoring function which facilitates the application circuit design, debugging and package thermal optimization. The voltage at TJ pin is directly proportional to the die temperature in Celsius (i.e., 250mV equals 25°C and 1.5V equals 150°C). The LT3692A is available in two packages: a 5mm × 5mm QFN and a 38-lead plastic TSSOP. Although Both include the full feature set, the TSSOP package enhances the thermal performance of the dual buck. The measured temperature of the LT3692A TSSOP die tops out at 80°C* with the two outputs each supporting 3A loads at 5V and 3.3V from an input voltage of 18V, L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 6V TO 32V 4.7μF ×2 VIN2 VIN1 SHDN1 BST1 6.8μH 0.47μF VOUT1 5V 3A fSW1 = 400kHz 47μF SW1 SW2 IND1 IND2 FB1 8.06k SS2 ILIM2 CMPI2 CMPO1 CMPO2 SS1 0.47μF 8.06k ILIM2 CLKOUT VC2 RT/SYNC GND 10.2k 24.9k VOUT2 3.3V 3A 47μF fSW2 = 400kHz SS2 ILIM1 VC1 680pF FB2 CMPI1 DIV 17.8k 6.8μH LT3692A VOUT2 VOUT1 42.2k 47pF SHDN2 BST2 TJ 0.1μF CLOCKOUT 400kHz 60.4k 330pF 0.1μF 33pF 36.5k dn492 F01 Figure 1. Dual 5V/3A/400kHz, 3.3V/3A/400kHz Application Keeps Temperature Rise Low at a VIN of 18V 07/11/492 with a switching frequency of 400kHz. Figure 1 shows the schematic of the measured application circuit. The same setup, but with 2.5A loads, drops the max die temperature to 68°C.* STANDALONE COMPARATOR BLOCK The LT3692A also includes a standalone comparator block, which provides a 720mV threshold with hysteresis and outputs an open-collector signal. This comparator can be configured as a power good flag signal by connecting CMPI pin to FB pin to monitor the output voltage. It can also be configured as a temperature flag, which gives a warning signal when the die temperature rises to a preset point. This function is realized together with the on-die temperature monitor. Figure 2 shows how to configure a 100°C temperature flag. OTHER FEATURES 180°. The phase difference on the LT3692A is adjusted by controlling the duty cycle of the synchronization signal. Frequency Division Frequency division makes it possible to tune the operating frequency of each channel to optimize overall performance and size. The frequency of channel 1 can be programmed to run at 1, 1/2, 1/4 or 1/8 the frequency of channel 2. Figure 3 shows the layout of a 3.3V/2.5A/550kHz channel and a 1.2V/1A/2.2MHz channel application. The relatively low 550kHz frequency of VOUT1 maximizes channel 1’s input voltage to 36V while meeting minimum on-time requirements and keeping the efficiency high. The high 2.2MHz frequency of VOUT2 allows the use of smaller components for channel 2 as shown in Figure 3. Despite the reduction in size, electrical and thermal performance is uncompromised. CONCLUSION Independent Adjustable Current Limit The switch current limit on each output can be programmed from 2A to 4.8A. This expands the number of loading combinations that can be safely implemented without risking thermal overload of the package under extreme conditions, such as a short-circuit. Likewise, the current limit can be used to protect the part in compact designs where the saturation margin on inductors is lowered to meet size constraints. The LT3692A is a dual output monolithic regulator that combines the ease-of-use and compact solution size of typical monolithic regulators with the flexibility of discrete, multichip solutions. Its high transient voltage capability, die temperature monitor, standalone comparator block, adjustable current limit, adjustable switching frequency and frequency division function and independent synchronization enable the LT3692A to work in many applications that other monolithic chips cannot. Independent Synchronization Independent synchronization allows any phase difference between the two outputs besides the standard 0° and VOUT2 3.3V VOUT2 100k 47μF CMPI2 CMPO2 100°C TEMP FLAG TJ GND 28k 10nF R3 73.2k dn492 F02 Figure 2. Temperature Flag Using Comparator Block and Temperature Monitor Figure 3. Dual 3.3V/2.5A/500kHz, 1.2V/1A/2.2MHz Layout. Channel 2 Requires Half of the Area of Channel 1 *TJ pin reading on a standard demo board (DC1403A) running in a 25°C ambient temperature environment. Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 3209 Linear Technology Corporation dn492f LT/TP 0711 226K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●