Datasheet LTC3404 (Analog Devices) - 9

LTC3404
U OPERATIO
1100
Slope Compensation and Inductor Peak Current
VIN = 3.3V Slope compensation provides stability in constant fre- 1000 quency architectures by preventing subharmonic oscilla- 900 tions at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current 800 signal at duty cycles in excess of 40%. As a result, the maximum inductor peak current is reduced for duty cycles 700 > 40%. This is shown in the decrease of the inductor peak current as a function of duty cycle graph in Figure 2. MAXIMUM INDUCTOR PEAK CURRENT (mA) 600 0 20 40 60 80 100 DUTY CYCLE (%) 3404 F02
Figure 2. Maximum Inductor Peak Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC3404 application circuit is shown on the first The inductor value also has an effect on Burst Mode page. External component selection is driven by the load operation. The transition to low current operation begins requirement and begins with the selection of L followed by when the inductor current peaks fall to approximately CIN and COUT. 250mA. Lower inductor values (higher ΔIL) will cause this to occur at lower load currents, which can cause a dip in
Inductor Value Calculation
efficiency in the upper range of low current operation. In The inductor selection will depend on the operating fre- Burst Mode operation, lower inductance values will cause quency of the LTC3404. The internal nominal frequency is the burst frequency to increase. 1.4MHz, but can be externally synchronized from 1MHz to
Inductor Core Selection
1.7MHz. Once the value for L is known, the type of inductor must be The operating frequency and inductor selection are inter- selected. High efficiency converters generally cannot related in that higher operating frequencies allow the use afford the core loss found in low cost powdered iron cores, of smaller inductor and capacitor values. However, oper- forcing the use of more expensive ferrite, molypermalloy, ating at a higher frequency generally results in lower or Kool Mμ® cores. Actual core loss is independent of core efficiency because of increased internal gate charge losses. size for a fixed inductor value, but it is very dependent on The inductor value has a direct effect on ripple current. The inductance selected. As inductance increases, core losses ripple current ΔIL decreases with higher inductance or go down. Unfortunately, increased inductance requires frequency and increases with higher VIN or VOUT. more turns of wire and therefore copper losses will increase. 1 ⎛ V ⎞ ΔI OUT L = ( V 1 Ferrite designs have very low core losses and are pre- f)(L) OUT − ⎝⎜ V (1) IN ⎠ ⎟ ferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Accepting larger values of ΔIL allows the use of smaller Ferrite core material saturates “hard,” which means that inductors, but results in higher output voltage ripple and inductance collapses abruptly when the peak design cur- greater core losses. A reasonable starting point for setting rent is exceeded. This results in an abrupt increase in ripple current is ΔIL = 0.4(IMAX). 3404fb 9