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4E02N-Rev.P001

800KHz Synchronous Step-up

Converter with 5.5A Switches

EC3689

Detailed Description

The EC3689 is based on a fixed frequency current mode pulse width modulation topology. The peak current of the

NMOS switch is sensed to limit the maximum current flowing through the switch and the inductor. The typical peak

current limit is set to 5.5A. An internal temperature sensor prevents the device from getting overheated in case of

excessive power dissipation.

Application Information

Because of the high integration of EC3689, the application circuit is simple. Only input capacitor CIN, output

Capacitor COUT, inductor L, output feedback resistors R3, R4 need to be selected for the targeted applications

specifications.

Setting the Output Voltage

The EC3689 output voltage can be adjusted with an external resistor divider (See Figure 1). The typical value of

the voltage on the FB pin is 500mV. The maximum allowed value for the output voltage is 5.5 V. Choose the

bottomresistor R4 in the 100k

Ω~500kΩ range to set the divider current at 1 µA or higher. The value of resistor R3,

Inductor Selection

The inductor DC current rating should be greater (by some margin) than the maximum input average current. The

highest peak current through the inductor and the switch depends on the output load, converter efficiency

η, the

input voltage (VIN), and the output voltage (VOUT). Estimation of the maximum average inductor current can be

done using Equation 2:

For example, for an output current of 2A at 5V with 90% efficiency, at least 3.8A of average current flows through

the inductor at a minimum input voltage of 2.9V.

The EC3689 step-up converters have been optimized to operate with an effective inductance in the range of 1µ H to

2.2µH and with output capacitors in the range of 20µ F to 100µ F. The internal compensation is optimized for an

performance of the device for specific operating conditions.

Input Capacitor

Place at least a 10 µF input ceramic capacitor close to the IC is to improve transient behavior of the regulator and

EMI behavior of the total power supply circuit.