AP61300/AP61302

Document number: DS43404 Rev. 3 - 2

13 of 21

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November 2022

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AP61300/AP61302

Application Information

1

Pulse Width Modulation (PWM) Operation

The AP61300/AP61302 device is a 2.4V-to-5.5V input, 3A output, fully integrated synchronous buck converter. Refer to the block diagram in

Figure 5. The device employs constant on-time control to provide fast transient response and easy loop stabilization. At the beginning of each

cycle, the one-shot pulse turns on the high-side power MOSFET, Q1, for a fixed on-time, tON. This one-shot on-pulse timing is calculated by the

converter’s input voltage and output voltage to maintain a pseudo-fixed frequency over the input voltage range. When Q1 is on, the inductor

current rises linearly and the device charges the output capacitor. Q1 turns off after the fixed on-time expires, and the low-side power MOSFET,

Q2, turns on. Once the output voltage drops below the output regulation, Q2 turns off. The one-shot timer is then reset and Q1 turns on again. The

on-time is inversely proportional to the input voltage and directly proportional to the output voltage. It is calculated by the following equation:

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Eq. 1

Where:



VIN is the input voltage



VOUT is the output voltage



fSW is the switching frequency

The off-time duration is tOFF and starts after the on-time expires. The off-time expires when the feedback voltage decreases below the reference

voltage, which then triggers the on-time duration to start again. The minimum off-time is 70ns typical.

2

Pulse Frequency Modulation (PFM) Operation

The AP61300/AP61302 can be programmed to enter PFM operation at light load conditions for high efficiency. During light load conditions, the

regulator automatically reduces the switching frequency. As the output current decreases, so too does the inductor current. The inductor current, IL,

eventually reaches 0A, marking the boundary between Continuous Conduction Mode (CCM) and Discontinuous Condition Mode (DCM). During

this time, both Q1 and Q2 are off, and the load current is provided only by the output capacitor. When VFB becomes lower than 0.6V, the next

cycle begins, and Q1 turns on. Because the AP61300/AP61302 can work in PFM during light load conditions, it can achieve power efficiency of up

to 84% at a 5mA load condition.

Likewise, as the output load increases from light load to heavy load, the switching frequency increases to maintain the regulation of the output

voltage. The transition point between light and heavy load conditions can be calculated using the following equation:

������������������������������ = (

������������������ − ������������������������

������������

) ∙ ������������������

Eq. 2

Where:



L is the inductor value

The quiescent current of AP61300/AP61302 is 19

μA typical under a no-load, non-switching condition.