AP65111A

Document number: DS39760 Rev. 1 - 2

8 of 13

www.diodes.com

June 2017

© Diodes Incorporated

AP65111A

Application Information

Theory of Operation

The AP65111A is a 1.5A current mode control, synchronous buck regulator with integrated power MOSFETs. Current mode control assures

excellent line regulation, load regulation, and a wide loop bandwidth for fast response to load transients. Figure 2 depicts the functional block

diagram of AP65111A.

The operation of one switching cycle can be explained as follows: The rising edge of the 500kHz oscillator clock signal sets the RS Flip-Flop. Its

output turns on HS MOSFET. When the HS MOSFET is on, inductor current starts to increase. The current sense amplifier with a gain of 0.22V/A

is used to detect the inductor current. Since the current mode control is subject to sub-harmonic oscillations that start at half duty cycle, ramp

slope compensation of 0.9V/T is added to the current sense signal. When the sum of the current sense amplifier output and the slope

compensation signal exceeds the EA output voltage, the RS Flip-Flop is reset and HS MOSFET is turned off.

Then synchr

onous LS MOSFET turns on until the next clock cycle begins. There is a “dead time” between the HS turn off and LS turn on that

prevents the switches from “shooting through” across the input supply to ground.

If the sum of the current sense amplifier output and the slope compensation signal does not exceed the EA output, then the falling edge of the

oscillator clock resets the Flip-Flop, and forces the HS MOSFET to turn off.

The voltage loop is compensated internally.

Enable

The enable (EN) input allows the user to control turning on or off the regulator. The AP65111A has an internal pull down resistor on the EN pin

and when the EN is not actively pulled up the part turns off.

Quiescent Current

Automated No-Load and Light-Load Operation

The AP65111A operates in light load high efficiency mode during low load current operation. The advantage of this light load efficiency mode is

lower power losses at no-load and light-load conditions. The AP65111A automatically detects the

inductor‟s valley current and enters the light load

high efficiency mode when value falls below zero Ampere. Once the inductor

‟s valley current exceeds this zero Ampere, the AP65111A transitions

from light load high efficiency mode to continuous PWM mode.

Current Limit Protection

In order to reduce the total power dissipation and to protect the application, AP65111A has cycle-by-cycle current limiting implementation. The

voltage drop across the internal high-side MOSFET is sensed and compared with the internally set current limit threshold. This voltage drop is

sensed at about 30ns after the HS turns on. When the peak inductor current exceeds the set current limit threshold, current limit protection is

activated. When the FB voltage pin dropped below 0.4V, the device enters Hiccup mode to periodically restart the part. This protection mode

greatly reduces the power dissipated on chip and reduces the thermal stress to help protect the device. AP65111A will exit Hiccup mode when the

over current situation is resolved.

Undervoltage Lockout (UVLO)

Undervoltage Lockout is implemented to prevent the IC from insufficient input voltages. The AP65111A has a UVLO comparator that monitors the

input voltage and the internal bandgap reference. If the input voltage falls below 4.05V, the AP65111A will disable. In this event, both HS and LS

MOSFETs are turned off.

Overvoltage Protection

When the AP65111A FB pin exceeds 115% of the nominal regulation voltage of 0.8V, the overvoltage comparator is tripped and internal regulator

Thermal Shutdown

The AP65111A has on-chip thermal protection that prevents damage to the IC when the die temperature exceeds safe margins. It implements a

thermal sensing to monitor the operating junction temperature of the IC. Once the die temperature rises to approximately +160°C, the thermal

protection feature gets activated. The internal thermal sense circuitry turns the IC off thus preventing the power switch from damage. A hysteresis

in the thermal sense circuit allows the device to cool down to approximately +120°C before the IC is enabled again through soft start. This thermal

hysteresis feature prevents undesirable oscillations of the thermal protection circuit.