LINEAR LT3663 User Guide

L DESIGN IDEAS

Supercapacitor Charger with Adjustable Output Voltage and Adjustable Charging Current Limit

by Jim Drew Introduction

For applications using larger value supercapacitors (tens to hundreds of VCAP

12V VIN VOUT

farads), a charger circuit with a rela-

LT3663 BUCK

tively high charging current is needed 50F

GND GND

to minimize the recharge time of the RUN

system. Supercapacitors are used as BIAS TOP RAIL +

GND CONTROL TOP ENA

energy hold up devices in applications

CIRCUIT

TOP CAP + TOP RAIL –

such as solid state RAID disks, where (FIGURE 3)

TOP CAP – BOT RAIL +

information stored in high speed vola- BOT CAP + BOT ENA

VIN VOUT BOT CAP – GND

tile memory must be transferred to

LT3663 BUCK

non-volatile flash memory when power 50F

GND GND GNDRUN GND

is lost. This transfer time may take minutes, requiring hundreds of farads to hold up the power supply until the transfer is complete. The requirement Figure 1. Block diagram for charging two supercapacitors in series for the recharge time of these banks of supercapacitors is typically less pin to ground. With its internal com- be held up, the minimum operating than one hour. To accomplish this, pensation network and internal boost voltage of the DC/DC converter sup- a high charging current is required. diode, the LT3663 requires a minimal porting the load, the distributed circuit This article describes a supercapacitor number of external components. resistances including the ESR of the charging circuit using the LT3663 that supercapacitors, and the required

meets these difficult requirements. Power Ride-Through hold up time. The LT3663 is a 1.2A, 1.5MHz step- Application Once the size of the supercapacitor

down switching regulator with output A procedure for selecting the size of is known, the charging current can current limit ideal for supercapacitor the supercapacitor is outlined in the be determined to meet the recharge applications. The part has an input September 2008 edition of Linear time requirements. The recharge voltage range of 7.5V to 36V,has Technology, in an article titled “Re- time (TRECHARGE) is the time required adjustable output voltage and adjust- place Batteries in Power Ride-Through to recharge the supercapacitors from able output current limit. The output Applications with Supercaps and the minimum operating voltage (VUV) voltage is set with a resistor divider 3mm × 3mm Capacitor Charger.” The of the DC/DC converter to the full network in the feedback loop while procedure determines the effective charge voltage (VFC) of the superca- the output current limit is set by a supercapacitor (CEFF) capacitance at pacitors. The voltage on the individual single resistor connected from the ILIM 0.3Hz, based on the power level to supercapacitors at the start of the

recharge cycle is the minimum oper-

VINVIN VOUT VOUT ating voltage divided by the number

IR05H40CSPTR 47µF

50V 1.2A (N) of supercapacitors in series. From

LT3663

16V here on, this article describes an ap-

BOOST

ENA RUN plication with two supercapacitors in

0.1µF 16V series. The recharge current (ICHARGE)

SW RFB1

L1 200k is determined by the capacitor charge 3.3µH

ILIM ISENSE control law:

RILIM GND FB

28.7k EFF FC UV

DFLS240

TCHARGE RECHARGE

This assumes that the voltage

L1: TDK VLCF5020T-3R3N1R7-1 86.6k

across the supercapacitor doesn’t discharge below the VUV/N value. This

Figure 2. Capacitor charger circuit using the LT3663 assumption is valid if the time period

Linear Technology Magazine • June 2009