LTC3406B

Part: LTC3406 — Linear Technology Corporation Type: Synchronous Step-Down Regulator Description: A high efficiency monolithic synchronous buck regulator with 2.5V to 5.5V input, 600mA output current, and 1.5MHz constant frequency operation.

Operating Conditions:

• Supply voltage: 2.5V to 5.5V
• Operating temperature: -40°C to 85°C
• Oscillator frequency: 1.5 MHz (typical)

Absolute Maximum Ratings:

• Max supply voltage: 6V
• P-Channel Switch Source Current (DC): 800mA
• N-Channel Switch Sink Current (DC): 800mA
• Max junction temperature: 125°C
• Max storage temperature: -65°C to 150°C

Key Specs:

• Regulated Feedback Voltage (VFB): 0.5850V to 0.6150V (-40°C ≤ TA ≤ 85°C)
• Regulated Output Voltage (LTC3406-1.5): 1.455V to 1.545V (IOUT = 100mA)
• Regulated Output Voltage (LTC3406-1.8): 1.746V to 1.854V (IOUT = 100mA)
• Input DC Bias Current Active Mode: 20 μA (typical, ILOAD = 0A)
• Input DC Bias Current Shutdown: 0.1 μA (typical, V RUN = 0V, V IN = 4.2V)
• Peak Inductor Current (IPK): 0.75A to 1.25A (VIN = 3V, VFB = 0.5V or VOUT = 90%, Duty Cycle < 35%)
• RDS(ON) of P-Channel FET: 0.4 Ω (typical, ISW = 100mA)
• RDS(ON) of N-Channel FET: 0.35 Ω (typical, ISW = -100mA)
• RUN Threshold (V RUN): 0.3V to 1.5V

Features:

• High Efficiency: Up to 96%
• Very Low Quiescent Current: Only 20 μA During Operation
• 600mA Output Current
• 1.5MHz Constant Frequency Operation
• No Schottky Diode Required
• Low Dropout Operation: 100% Duty Cycle
• 0.6V Reference Allows Low Output Voltages
• Shutdown Mode Draws ≤ 1 μA Supply Current
• Current Mode Operation for Excellent Line and Load Transient Response
• Overtemperature Protected
• Low Profile (1mm) ThinSOT Package

Applications:

• Cellular Telephones
• Personal Information Appliances
• Wireless and DSL Modems
• Digital Still Cameras
• MP3 Players
• Portable Instruments

Package:

• S5 Package: 5-LEAD PLASTIC TSOT-23

• ■ High Efficiency: Up to 96%
• ■ Very Low Quiescent Current: Only 20 μ A During Operation
• ■ 600mA Output Current
• ■ 2.5V to 5.5V Input Voltage Range
• ■ 1.5MHz Constant Frequency Operation
• ■ No Schottky Diode Required
• ■ Low Dropout Operation: 100% Duty Cycle
• ■ 0.6V Reference Allows Low Output Voltages
• ■ Shutdown Mode Draws ≤ 1 μ A Supply Current
• ■ Current Mode Operation for Excellent Line and Load Transient Response
• ■ Overtemperature Protected
• ■ Low Profile (1mm) ThinSOT TM Package

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are T A = 25 ° C. VIN = 3.6V unless otherwise specified.

3406fa

In most applications the LTC3406 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150 ° C, both power switches will be turned off and the SW node will become high impedance.

To avoid the LTC3406 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by:

where P D is the power dissipated by the regulator and θ JA is the thermal resistance from the junction of the die to the ambient temperature.

The junction temperature, T J , is given by:

where T A is the ambient temperature.

As an example, consider the LTC3406 in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70 ° C. From the typical performance graph of switch resistance, the R DS(ON) of the P-channel switch at 70 ° C is approximately 0.52 Ω . Therefore, power dissipated by the part is:

For the SOT-23 package, the θ JA is 250 ° C/W. Thus, the junction temperature of the regulator is:

which is below the maximum junction temperature of 125 ° C.

Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (R DS(ON) ).