Datasheet LTC3408 (Analog Devices) - 7

LTC3408
U OPERATIO (Refer to Functional Diagram)
off and on the bypass P-channel MOSFET with a frequency but less than VIN/3, the bypass P-channel MOSFET will be of approximately 50kHz to 100kHz at 1.6A peak current. on, but the main switch will be off. For best performance This will continue until the short is removed. While the and lowest voltage drop from VIN to VOUT, always ensure bypass P-channel MOSFET is pulsing intermittently, the that the REF voltage is greater than both 1.2V and VIN/3. inherent current limit of the step-down regulator limits its An important detail to remember is that at low input peak current to about 1A. supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics).
Dropout Operation
Therefore, the user should calculate the power dissipa- If the reference voltage would cause VOUT to exceed VIN, tion when the LTC3408 is used at 100% duty cycle with the LTC3408 enters dropout operation. During dropout, low input voltage (See Thermal Considerations in the the main switch remains on continuously and operates at Applications Information section). 100% duty cycle. If the voltage at REF is less than 1.2V, the bypass P-channel MOSFET will stay off even in dropout
Low Supply Operation
operation. The output voltage is then determined by the The LTC3408 will operate with input supply voltages as input voltage minus the voltage drop across the main switch low as 2.5V, but the maximum allowable output current is and the inductor. If the voltage at REF is greater than 1.2V, reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input 1200 voltage for various output voltages. 1000
Slope Compensation and Inductor Peak Current
VOUT = 1.8V 800 VOUT = 2.5V Slope compensation provides stability in constant fre- VOUT = 1.5V 600 quency architectures by preventing subharmonic oscilla- tions at high duty cycles. It is accomplished internally by 400 adding a compensating ramp to the inductor current 200 signal at duty cycles in excess of 40%. Normally, this MAXIMUM OUTPUT CURRENT (mA) results in a reduction of maximum inductor peak current 02.5 3.0 3.5 4.0 4.5 5.0 5.5 for duty cycles > 40%. However, the LTC3408 uses a SUPPLY VOLTAGE (V) patent-pending scheme that counteracts this compensat- 3408 F02 ing ramp, which allows the maximum inductor peak
Figure 2. Maximum Output Current vs Input Voltage
current to remain unaffected throughout all duty cycles.
U U W U APPLICATIO S I FOR ATIO
The basic LTC3408 application circuit is shown in Fig- currents. As Equation 1 shows, a greater difference be- ure 1. External component selection is driven by the load tween VIN and VOUT produces a larger ripple current. requirement and begins with the selection of L followed by Where these voltages are subject to change, the highest CIN and COUT. VIN and lowest VOUT will determine the maximum ripple current. A reasonable starting point for setting ripple
Inductor Selection
current is IL = 120mA (20% of the maximum load, 600mA). For most applications, the value of the inductor will fall in the range of 4µH to 6µH. Its value is chosen based on the 1  V  ∆I OUT L = VOUT 1– desired ripple current. Large value inductors lower ripple (1) f ( ) L ( )  VIN  current and small value inductors result in higher ripple 3408f 7