Datasheet LTC3403 (Analog Devices) - 9

LTC3403
U OPERATIO (Refer to Functional Diagram)
1200
Slope Compensation and Inductor Peak Current
1000 Slope compensation provides stability in constant fre- VOUT = 1.8V quency architectures by preventing subharmonic oscilla- 800 VOUT = 2.5V tions at high duty cycles. It is accomplished internally by VOUT = 1.5V 600 adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this 400 results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the LTC3403 uses a 200 MAXIMUM OUTPUT CURRENT (mA) patent-pending scheme that counteracts this compensat- 0 ing ramp, which allows the maximum inductor peak 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) current to remain unaffected throughout all duty cycles. 3403 F02
Figure 2. Maximum Output Current vs Input Voltage U U W U APPLICATIO S I FOR ATIO
The basic LTC3403 application circuit is shown in Fig- The DC current rating of the inductor should be at least ure␣ 1. External component selection is driven by the load equal to the maximum load current plus half the ripple requirement and begins with the selection of L followed by current to prevent core saturation. Thus, a 720mA rated CIN and COUT. inductor should be enough for most applications (600mA + 120mA). For better efficiency, choose a low DC-resis-
Inductor Selection
tance inductor. For most applications, the value of the inductor will fall in The inductor value also has an effect on Burst Mode the range of 1µH to 4.7µH. Its value is chosen based on the operation. The transition to low current operation begins desired ripple current. Large value inductors lower ripple when the inductor current peaks fall to approximately current and small value inductors result in higher ripple 200mA. Lower inductor values (higher IL) will cause this currents. As Equation 1 shows, a greater difference be- to occur at lower load currents, which can cause a dip in tween VIN and VOUT produces a larger ripple current. efficiency in the upper range of low current operation. In Where these voltages are subject to change, the highest Burst Mode operation, lower inductance values will cause VIN and lowest VOUT will determine the maximum ripple the burst frequency to increase. current. A reasonable starting point for setting ripple current is IL = 240mA (40% of the maximum load, 600mA).
Inductor Core Selection
Different core materials and shapes will change the size/ 1  V  ∆I OUT current and price/current relationship of an inductor. L = VOUT 1– (1) f ( ) L ( )  VIN  Toroid or shielded pot cores in ferrite or permalloy mate- rials are small and don’t radiate much energy but generally At output voltages below 0.6V, the switching frequency cost more than powdered iron core inductors with similar decreases linearly to a minimum of approximately 700kHz. electrical characteristics. The choice of which style induc- This places the maximum ripple current (in forced con- tor to use often depends more on the price versus size tinuous mode) at the highest input voltage and the lowest requirements and any radiated field/EMI requirements output voltage. In practice, the resulting ouput ripple than on what the LTC3403 requires to operate. Table 1 voltage is 10mV to 15mV using the components specified shows some typical surface mount inductors that work in Figure 1. well in LTC3403 applications. 3403f 9