Datasheet LTC3412A (Analog Devices) - 10

LTC3412A
APPLICATIONS INFORMATION Slope Compensation and Inductor Peak Current
Although frequencies as high as 4MHz are possible, the Slope compensation provides stability in constant fre- minimum on-time of the LTC3412A imposes a minimum quency architectures by preventing subharmonic oscilla- limit on the operating duty cycle. The minimum on-time tions at duty cycles greater than 50%. It is accomplished is typically 110ns; therefore, the minimum duty cycle is internally by adding a compensating ramp to the inductor equal to 100 • 110ns • f(Hz). current signal at duty cycles in excess of 40%. Normally,
Inductor Selection
the maximum inductor peak current is reduced when slope compensation is added. In the LTC3412A, however, For a given input and output voltage, the inductor value slope compensation recovery is implemented to keep the and operating frequency determine the ripple current. The maximum inductor peak current constant throughout the ripple current ∆IL increases with higher VIN or VOUT and range of duty cycles. This keeps the maximum output decreases with higher inductance. current relatively constant regardless of duty cycle. ⎛ V ⎞ ⎛ V ⎞ ΔI OUT OUT L =
Short-Circuit Protection
⎝⎜ fL ⎠⎟ 1– ⎝⎜ VIN ⎠⎟ When the output is shorted to ground, the inductor cur- Having a lower ripple current reduces the core losses in rent decays very slowly during a single switching cycle. the inductor, the ESR losses in the output capacitors, and To prevent current runaway from occurring, a secondary the output voltage ripple. Highest efficiency operation is current limit is imposed on the inductor current. If the achieved at low frequency with small ripple current. This, inductor valley current increases larger than 4.4A, the top however, requires a large inductor. power MOSFET will be held off and switching cycles will A reasonable starting point for selecting the ripple current be skipped until the inductor current is reduced. is ∆IL = 0.4(IMAX). The largest ripple current occurs at the The basic LTC3412A application circuit is shown in Fig- highest VIN. To guarantee that the ripple current stays ure 1. External component selection is determined by the below a specified maximum, the inductor value should maximum load current and begins with the selection of be chosen according to the following equation: the operating frequency and inductor value followed by ⎛ V ⎞ ⎛ ⎞ OUT VOUT C L = IN and COUT. ⎜ f ⎟ 1– ⎜ ⎟ ⎝ ΔI V L(MAX) ⎠ ⎝ IN(MAX) ⎠
Operating Frequency
The inductor value will also have an effect on Burst Mode Selection of the operating frequency is a trade-off between operation. The transition to low current operation begins efficiency and component size. High frequency operation when the peak inductor current falls below a level set by allows the use of smaller inductor and capacitor values. the burst clamp. Lower inductor values result in higher Operation at lower frequencies improves efficiency by ripple current which causes this to occur at lower load reducing internal gate charge losses but requires larger currents. This causes a dip in efficiency in the upper inductance values and/or capacitance to maintain low range of low current operation. In Burst Mode operation, output ripple voltage. lower inductance values will cause the burst frequency The operating frequency of the LTC3412A is determined to increase. by an external resistor that is connected between pin RT and ground. The value of the resistor sets the ramp current
Inductor Core Selection
that is used to charge and discharge an internal timing Once the value for L is known, the type of inductor must capacitor within the oscillator and can be calculated by be selected. Actual core loss is independent of core size using the following equation: for a fixed inductor value, but it is very dependent on the inductance selected. As the inductance increases, core 3.08 • 1011 R (Ω)–10kΩ losses decrease. Unfortunately, increased inductance OSC = f requires more turns of wire and therefore copper losses will increase. 3412aff 10 Fo F r o rmo m r o e r e inf n o f r o m r a m t a iton o n ww w w w . w line n a e r a .rco c m o / m L / T L C T 3 C 4 3 1 4 2 1 A 2 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts