AP64100Q

Document number: DS43575 Rev. 1 - 2

15 of 26

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July 2021

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AP64100Q

Application Information (continued)

11

Adjusting Switching Frequency (continued)

RT

RT/CLK

External

Clock

Oscillator

with PLL

CLK

Figure 27. Switching Between Resistor Timing and External Clock Synchronization Modes

In applications where both Resistor Timing and External Clock Synchronization modes are required, the device can be configured as shown in

Figure 27. Before an external clock signal is available at the RT/CLK pin, the device operates in Resistor Timing mode. When an external clock is

supplied to the RT/CLK pin, the device automatically transitions from Resistor Timing mode to External Clock Synchronization mode typically

within

85μs. When the external clock signal is disconnected from the RT/CLK pin, the device’s switching frequency returns to being set in Resistor

Timing mode. When switching between Resistor Timing and External Clock Synchronization modes, it is recommended that the external clock

signal is within ±25% of the frequency controlling the device in Resistor Timing mode to prevent large changes in switching frequency within the

device.

12

Inductor

Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate

the inductor value:

������ =

������������������������ ∙ (������������������ − ������������������������)

������������������ ∙ ∆������������ ∙ ������������������

Eq. 8

Where:



∆IL is the inductor current ripple



fSW is the buck converter switching frequency

For the AP64100Q

, choose ∆IL to be 30% to 40% of the maximum load current of 1A.

The inductor peak current is calculated by:

������������

������������������������ = ������������������������������ +

∆������������

������

Eq. 9

Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the inductor decreases the

converter efficiency while increasing the temperatures of the inductor and the internal power MOSFETs. Therefore, choosing an inductor with the

appropriate saturation current rating is important. For most applications, it is recommended to select an inductor of approximately 6.8µH to 33µH

with a DC current rating of at least 35% higher than the maximum load current. For highest efficiency, the inductor’s DC resistance should be less

than 5

0mΩ. Use a larger inductance for improved efficiency under light load conditions.