Catalogs >
ADI >
LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO
24Pages

Catalog excerpts

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 1

LT3645 1 3645f LOAD CURRENT (mA) 0 100 50 EFFICIENCY (%) 60 70 80 90 200 300 3645 TA01b 400 500 VOUT = 3.3V VIN = 12V VOUT = 5V TYPICAL APPLICATION FEATURES DESCRIPTION 36V 500mA Step-Down Regulator and 200mA LDO The LT®3645 is a dual output regulator combining a 500mA buck regulator and a 200mA low dropout linear regulator (LDO). The wide input voltage range of 3.6V to 36V makes the LT3645 suitable for regulating power from a wide variety of sources, including 24V industrial supplies and automotive batteries. Its high operating frequency allows the use of tiny, low cost inductors and capacitors, resulting in a very small solution. Cycle-by-cycle current limit and frequency foldback provide protection against shorted outputs. Soft-start and frequency foldback eliminate input current surge during start-up. The linear regulator operates from the VCC2 pin at voltages down to 1.2V. It supplies 200mA of output current with a typical dropout voltage of 310mV. Other features of the LT3645 include a <2ìA shutdown, short circuit protection, soft-start and thermal shutdown. The LT3645 is available in the thermally enhanced 16-lead (3mm × 3mm) QFN package, or a 12-lead MSE package. 3.3V/5V Step-Down Converter APPLICATIONS n Automotive CMOS Image Sensors n Industrial/Automotive Micro-Controller Supply L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. n Wide Input Range: Operation from 3.6V to 36V Overvoltage Lockout Protects Circuit Through 55V Transients on Input n 500mA Output Current Switching Regulator n High Switching Frequency: 750kHz n 200mA Low Dropout Linear Regulator 1.2V to 16V Input; 0.8V to 8V Output 310mV Dropout Voltage VCC2 to OUT2 n Precision Programmable Undervoltage Lockout n Short-Circuit Robust n Internal Soft-Start n <2ìA Shutdown Current n Small Thermally Enhanced 16-Lead (3mm × 3mm) QFN and 12-Lead MSE Packages Buck Regulator Effi ciency 1ìF 3645 TA01a PGOOD 6.2V TO 36V GND SW DA FB FB2 EN2 VCC2 VIN EN/UVLO NPG OUT2 10ìF 5V 300mA 3.3V 200mA 15ìH 52.3k 31.6k 10k 10k BOOST LT3645 2.2ìF 0.1ìF ON OFF

Open the catalog to page 1

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 3

LT3645 3 3645f ELECTRICAL CHARACTERISTICS PARAMETER CONDITIONS MIN TYP MAX UNITS Undervoltage Lockout on VIN Rising l 3 3.4 3.6 V Overvoltage Lockout on VIN Rising l 36 38.5 40 V Overvoltage Lockout Hysteresis 1 V Feedback Voltage FB l 0.79 0.785 0.8 0.8 0.81 0.813 V FB Pin Bias Current l 20 300 nA Feedback Voltage Line Regulation l 0.015 %/V VIN Quiescent Current Not Switching 1.4 3 mA VIN Quiescent Current in Shutdown VEN/UVLO = 0.3V, VCC2 = 0V, VOUT2 = 0V 0.01 2 ìA Switching Frequency 675 750 825 kHz Maximum Duty Cycle 100mA Load l 83 87 % Switch Current Limit Rising (Note 4) 0.8 1 1.25...

Open the catalog to page 3

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 4

LT3645 4 3645f Buck Minimum Input Voltage, VOUT = 3.3V Effi ciency VOUT = 5V Effi ciency VOUT = 3.3V Buck Minimum Input Voltage, VOUT = 5V PARAMETER CONDITIONS MIN TYP MAX UNITS FB NPG Threshold, % of Regulation Voltage VFB2 = 800mV, VFB Rising 88 90 92 % NPG Threshold Hysteresis 25 mV E LECTRICAL CHARACTERISTICSThe l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless otherwise noted. (Notes 2, 3) Note1: Stresses beyond those listed under Absolute Maximum...

Open the catalog to page 4

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 7

LT3645 7 3645f PIN FUNCTIONS EN/UVLO (Pin 1/Pin 5): The EN/UVLO pin is used to enable the buck switching regulator and the low dropout linear regulator (LDO). An accurate threshold of 1.23V allows the user to set the undervoltage lockout point with a simple resistor divider, see Precision Undervoltage Lockout section for more information. The EN/UVLO pin can be tied directly to VIN if the UVLO or shutdown is not used. FB (Pin 2/Pin 6): The FB pin programs the buck output voltage. The LT3645 regulates the FB pin to 0.8V. The feedback resistor divider tap should be connected to this pin. The...

Open the catalog to page 7

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 8

LT3645 8 3645f BLOCK DIAGRAM VIN EN/UVLO START 1.23V 1.3V START BUCK START LDO ERROR AMP CURRENT COMPARATOR VC BUCK DRIVER LOGIC –40mV BOOST 0.8V REFERENCE PGOOD C1 EN2 NPG GND – + – + – + + – + + OSCILLATOR SOFT-START S Q R – + Q1 SW DA GND C2 L1 D1 R1 R2 C4 R3 R4 C3 FB VCC2 VIN OUT2 LDO DRIVER ERROR AMP 0.797V 0.72V FB2 Q2 SOFT-START – + – + LDO ON ON OFF SLOPE COMPENSATION

Open the catalog to page 8

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 9

LT3645 9 3645f OPERATION The LT3645 includes a constant frequency, current mode step-down buck switching regulator together with a lowdropout regulator (LDO). If EN/UVLO is less than ~0.7V, both the buck and LDO are off, the output is disconnected and the input current is less than 2ìA. The buck turns on when EN/UVLO is greater than 1.23V. An undervoltage lockout (UVLO) turns the buck and LDO off when VIN is less than 3.4V. An overvoltage lockout (OVLO) turns the buck and LDO off when VIN is greater than 38.5V. The part will withstand nonrepetitive one second input voltage transients up to...

Open the catalog to page 9

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 11

LT3645 11 3645f APPLICATIONS INFORMATION Now the required on time has decreased below the minimum on time of 100ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the part skips a few pulses so that the average inductor current meets and does not exceed the load current requirement. The LT3645 is robust enough to survive prolonged operation under these conditions as long as the peak inductor current does not exceed 1.2A. Inductor saturation due to high current may further limit performance in this operating region. Inductor Selection and...

Open the catalog to page 11

LT3645 - 36V 500mA Step-Down Regulator and 200mA LDO - 12

LT3645 12 3645f APPLICATIONS INFORMATION Output Capacitor The output capacitor has two essential functions. Along with the inductor, it fi lters the square wave generated by the LT3645 to produce the DC output. In this role it determines the output ripple so low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3645’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good value is: COUT = 26.4/(VOUT • ƒ) where f is the...

Open the catalog to page 12