Know About LTC3355 IC Pin Description and Its Operation

Some critical applications need temporary hold off supply voltage for the backup supply kicks especially the applications like power meters, power electronic drives and industrial or home alarm systems. Therefore, the LTC 3355 voltage regulator IC supplies momentary uninterrupted power to the device from the energy stored in the capacitor and also it produces regulated power. It thus reduces the need to replace a battery and minimizes the maintenance of portable backup systems and low-cost applications. This article provides a brief description about this IC pin, its configuration, and operation.

What is LTC 3355 IC?

Linear Technology Corporation introduced an LTC3355 voltage regulator IC in March 2014 which is a complete ride-through DC to DC system IC. It features a buck regulator with built-in boost converter for a short period backup of Vout from a single super capacitor energy source during a sudden loss of Vin power. The functions of this IC include charging of super capacitor or other storage elements, monitoring of Vin, VCAP, and Vout and switch over to automatic backup power.

This IC can supply 1Ampere at 2.7 to 5V due to the external resistor divider. The LTC 3355 IC charges the supercapacitor as VCAP voltage, and if Vin drops to a certain limit (approximately 0.5V), this voltage powers the boost regulator so that it supplies the same Vout automatically. It uses 1A programmable constant voltage, constant current linear charger to charge a single super capacitor– NiMH battery or electrolytic capacitor powered from Vout. It is housed in a 20-lead 4 × 4 mm QFN surface package, and its operating temperature range is between 40 to 125-degree centigrade junction temperatures.

Features of LTC 3355 Voltage Regulator IC

• Voltage Range Vin: 3V to 20V
• Voltage Range Vout: 2.7V to 5V
• 5Ampere Boost Backup Regulator Powered from Single Supercapacitor
• 1Ampere Current-Mode Buck Main Regulator
• Programmable VIN Current Limit
• Boost Regulator Operates Down to 0.5V for Maximum Utilization of Supercapacitor Energy
• VOUT Power on Reset Output
• Programmable Boost Current Limit
• VIN Power Fail Indicator
• VCAP Power Good Indicator
• Charger Supports Single Cell CC/CV Battery Charging
• Compact 20-Lead 4mm x 4mm x 0.75mm QFN Package

Pin Configuration of LTC 3355

Pin 1(PFI): It is an input to a Power-Fail Comparator. It can be programmed such that connecting this pin to a voltage divider between the VIN and ground cause the input voltage below which the PFOB pin indicates a power-fail condition.

Pin 2 (FB): The buck and boost voltage control operation VOUT is decided by this PIN via an external resistor divider and its reference voltage is 0.8V.

Pin 3 (MODE): The buck and boost switching modes like PWM and Burst modes are selected by this PIN, where low is PWM mode and high is Burst Mode operation.

Pin 4 (VINS): This is an input current limit sense voltage pin, which connects a sense resistor from VINS to VIN. It must be locally bypassed with a low ESR ceramic capacitor or connected to VIN if input current limit is not needed.

Pin 5 (VIN): It is an input power pin that supplies current to a buck power switch and an internal regulator, and it must be bypassed with a low ESR ceramic capacitor.

Pin 6 (VINM5): This filters the voltage of VIN – 4.65 to an internal supply regulator with a 1μF ceramic capacitor from VINM5 to VIN.

Pin 7 (SW1): It is a buck output of the internal power switch. Catch diode and inductor are connected to this pin to reduce EMI.

Pin 8 (EN_CHG): When this pin goes high, it enables the super capacitor charger.

Pin 9 (CPGOOD): This Open-drain output pin is high when the VCAP voltage is about or higher than 92.5% of the programmed voltage.

Pin 10 (PFOB): It is an open drain of the Power-Fail Comparator. When the PFI input determines input supply drop out, thegroundthis pin switches low and enables the boost converter.

Pin 11 (CFB): This pin programs the VCAP voltage using external resistor divider and its reference voltage is 0.8V.

Pin 12 (ICHG): This pin decides the VCAP charge current by connecting a rest ground.

Pin 13 (RSTB): It is an Open-drain reset output which becomes high when the VOUT voltage is more than 92.5% of the programmed regulation voltage.

Pin 14 (VCAP): This pin is connected to a supercapacitor so that it gives constant-current, constant-voltage, linear-charger output.

Pin 15 (VOUT): This is an output voltage supply. It supplies the buck power from the VIN when the input voltage is present and the boost power from VCAP when the input voltage drops out.

Pin 16, 17 (SW2): This is a boost output of the internal power switch, and these pins are connected to the rectifier diode and inductor to reduce EMI.

Pin 18 (INTVCC): This pin filters an internal supply using the 1μF ceramic capacitor between this pin to ground. This INTVCC pin is at 2.5V during start up until VOUT exceeds 2.5V, and then it follows VOUT.

Pin 19 (VCBST): The output of the boost internal error amplifier is connected to this pin, and the potential at this pin controls the peak switch current for the boost regulator. For achieving compensation of boost control loop, connect an RC series network from this pin to the ground.

Pin 20 (IBSTPK): This pin connects the resistor to the ground to set the boost peak current limit.

Pin 21 (Exposed Pad or GND): It is a Ground pin. For achieving thermal conduction, the exposed pad must be connected to a continuous ground plane on the printed circuit board’s second layer.

Operation of LTC 3355 Voltage Regulator IC

During startup voltage, onlyVIN is available since the VCAP and VOUT are at zero potential. From this VIN, the internal 2.5V regulator powers INTVCC, and this INTVCC powers all of the low-voltage circuits. Till the feedback voltage is 0.8V, the buck regulator drives VOUT positive through an inductor.After the VOUT becomes more than 2.5V, the INT VCC tracks VOUT, and the supply for all the internal circuits is supplied from the VOUT instead of VIN.

The LTC3355 uses buck regulator of the MHz constant frequency with an internal slope compensation that controls the voltage at VOUT. An error amplifier in this regulator IC compares a reference voltage of 0.8V to the divided output voltage at FB. For optimizing efficiency, the burst mode operation can be selected at low-load currents using the MODE pin. When the MODE pin is low,t he buck is in Pulse Width Modulation (PWM) mode – and,i f it is high, then the burst mode operates. The buck input voltage range is 3V to 20V which is set by using internal compensator circuit.

A monolithic 1 MHz constant frequency boost regulator is enabled with an internal compensation when the VIN is not available after enabling the PFI pin to disable the buck regulator. VOUT voltage is regulated by this boost regulator using the stored voltage of VCAP. For instance, the above error amplifier compares FB divided output voltage with 0.8V reference voltage by adjusting current through the peak inductor. The range of boost regulator is 0.5V to 5V, and the peak boost current range is 1A to 5A, which is set by IBSTPK pin for low-current applications. This is also operated in two modes: PWM mode and burst modes by the MODE pin enable and disable states.

An internal constant current or constant voltage linear charger of 1 Ampere rating charges the super capacitor by supplying current from the VOUT to the VCAP. The three conditions that are met to enable this charging process are VIN is higher than the programmable voltage through the pin PFI, VOUT is in regulation mode, and the EN_CHG pin is high. The ICHG pin resistor determines the charging current and the internal amplifier sets this ICHG voltage to 0.8V to create the reference current. When the CFB voltage is equal to a 0.8V reference voltage, hysteretic comparator turns the charger off and the charger will be turned on when the CFB voltage falls to 30mV below the CFB reference voltage.

Due to these wide variety of characteristics,these voltage regulator ICs are used in buck start, boost rectifier and frequency compensation, high-temperature and low-ripple applications. The given information is in a limited form, which is in brief; therefore, for deep information, you can download its data sheet. We hope that the given information is worthy of your valuable suggestions, feedback and comments, and therefore, write to us in the comments section below.

Photo Credits

• Voltage regulator IC by linearnews
• LTC 3355 IC by linearnews
• Pin Configuration of LTC 3355 by linear
• Operation of LTC 3355 IC by linear
• LTC 3355 IC Characteristics by linear