AP6503

Document number: DS35077 Rev. 7 - 2

8 of 16

www.diodes.com

May 2017

© Diodes Incorporated

AP6503

Application Information

Theory of Operation

The AP6503 is a 3A current mode control, synchronous buck regulator with built-in power MOSFETs. Current mode control assures excellent

line and load regulation and a wide loop bandwidth for fast response to load transients. Figure 3 depicts the functional block diagram of AP6503.

The operation of one switching cycle can be explained as follows. At the beginning of each cycle, HS (high-side) MOSFET is off. The EA output

voltage is higher than the current sense amplifier output, and the current comparator’s output is low. The rising edge of the 340kHz oscillator

clock signal sets the RS Flip-Flop. Its output turns on HS MOSFET. The current sense amplifier is reset for every switching cycle.

When the HS MOSFET is on, inductor current starts to increase. The Current Sense Amplifier senses and amplifies the inductor current. Since

the current mode control is subject to sub-harmonic oscillations that peak at half the switching frequency, Ramp slope compensation is utilized.

This will help to stabilize the power supply. This Ramp compensation is summed to the Current Sense Amplifier output and compared to the

Error Amplifier output by the PWM Comparator. 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.

For one whole cycle, 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. The output of the Error Amplifier increases when feedback voltage (VFB) is lower than

the reference voltage of 0.925V. This also increases the inductor current as it is proportional to the EA voltage.

If in one cycle the current in the power MOSFET does not reach the COMP set current value, the power MOSFET will be forced to turn off. When

the HS MOSFET turns off, the synchronous 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 swit

ches from “shooting through” from the input supply to ground.

The voltage loop is compensated through an internal transconductance amplifier and can be adjusted through the external compensation

components.

Enable

Above the ‘EN Rising Threshold’, the internal regulator is turned on and the quiescent current can be measured above this threshold. The enable

(EN) input allows the user to control turning on or off the regulator. To enable the AP6503, EN must be pulled above the ‘EN Lockout Threshold

Voltag

e’ and to disable the AP6503, EN must be pulled below ‘EN Lockout Threshold Voltage - EN Lockout Hysteresis’ (2.2V-0.22V =1.98V).

External Soft Start

Soft start is traditionally implemented to prevent the excess inrush current. This in turn prevents the converter output voltage from overshooting

when it reaches regulation. The AP6503 has an internal current source with a soft start capacitor to ramp the reference voltage from 0V to

0.925V. The soft start current is 6µA. The soft start sequence is reset when there is a Thermal Shutdown, Undervoltage Lockout (UVLO) or when

the part is disabled using the EN pin.

External Soft Start can be calculated from the formula below:

DT

DV

*

C

SS

I



Where:

C = External Capacitor

DV=change in feedback voltage from 0V to maximum voltage

DT = Soft Start Time

Current Limit Protection

In order to reduce the total power dissipation and to protect the application, AP6503 has a 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. Refer to the block diagram in figure 2. The peak inductor current plus the slope compensation ramp

exceeds the set current limit threshold, current limit protection is activated. Due to the slope compensation contribution to the protection, the

overall current limit changes with duty cycle. Figures 3, 4, and 5 below showed the typical peak current limit versus output voltage for 15V, 20V,

and 23V input respectively for high duty cycle. The current limit variation is not as significant with duty cycle less than 50% due to smaller slope

compensation magnitude. During this time, the feedback voltage (VFB) drops down. When the voltage at the FB pin reaches 0.3V, the internal

oscillator shifts the frequency from the normal operating frequency of 340Khz to a fold-back frequency of 100Khz. The current limit is reduced to

70% of nominal current limit when the part is operating at 100Khz. The low Fold-back frequency prevents uncontrollable runaway current.