AP65211

Document number: DS39118 Rev. 3 - 3

9 of 14

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January 2018

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AP65211

Application Information (Continued)

Current Limit Protection

In order to reduce the total power dissipation and to protect the application, AP65211 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. AP65211 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 AP65211 has a UVLO comparator that monitors the

input voltage and the internal bandgap reference. If the input voltage falls below 4.05V, the AP65211 will latch the undervoltage fault. In this

event, the output will be pulled low and power has to be re-cycled to reset the UVLO fault.

Overvoltage Protection

When the AP65211 FB pin exceeds 115% of the nominal regulation voltage of 0.8V, the overvoltage comparator is tripped and internal regulator

would stop switching. The VOUT would stay high voltage as tripped point and slowly discharged by output capacitance.

Thermal Shutdown

The AP65211 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.

Setting the Output Voltage

The output voltage can be adjusted from 0.8V using an external resistor divider. Table 1 shows a list of resistor selection for common output

voltages. A serial resistor RT is also recommended for improving the system stability, especially for low VOUT (<3.3V). An optional CFF of 10pF

to 470pF used to boost the phase margin. Resistor R1 is selected based on a design tradeoff between efficiency and output voltage accuracy.

For high values of R1 there is less current consumption in the feedback network. R1 can be determined by the following equation:



















1

0.8

V

R

R

OUT

2

1

Vout

FB

R1

R2

RT

CFF

Figure 2 Feedback Divider Network

Table 1 Recommended Component Selection

VOUT (V)

R1 (kΩ)

R2 (kΩ)

RT (kΩ)

L1 (µH)

1.05

10

32.4

300

1.5

1.2

15

30.1

249

1.5

1.8

40.2

32.4

120

2.2

2.5

40.2

19.1

100

2.2

3.3

40.2

13

75

4.7

5

40.2

7.68

75

6.5