Data Sheet

ADP2109

Rev. B | Page 11 of 16

ENABLE/SHUTDOWN

The ADP2109 starts operation with soft start when the EN pin

is toggled from logic low to logic high. Pulling the EN pin low

forces the device into shutdown mode, reducing the shutdown

current below 1 μA.

DISCHARGE SWITCH

The ADP2109 has an integrated resistor of typically 150 Ω, as

shown in Figure 27, to discharge the output capacitor when the

EN pin goes low or when the device goes into under-voltage

lock out or thermal shutdown. The time to discharge is typically

200 µs.

FB

Figure 27. Internal Discharge Switch on Feedback

SHORT-CIRCUIT PROTECTION

The ADP2109 includes frequency foldback to prevent output

current runaway on a hard short. When the voltage at the feed-

back pin falls below half of the target output voltage, indicating

the possibility of a hard short at the output, the switching fre-

quency is reduced to half of the internal oscillator frequency.

The reduction in the switching frequency allows more time

for the inductor to discharge, preventing a runaway of output

current.

UNDERVOLTAGE LOCKOUT

To protect against battery discharge, undervoltage lockout

circuitry is integrated on the ADP2109. If the input voltage

drops below the 2.15 V undervoltage lockout (UVLO) thre-

shold, the ADP2109 shuts down and both the power switch

and synchronous rectifier turn off. When the voltage rises

above the UVLO threshold, the soft start period is initiated,

and the part is enabled.

THERMAL PROTECTION

In the event the ADP2109 junction temperatures rise above 150°C,

the thermal shutdown circuit turns off the converter. Extreme

junction temperatures can be the result of high current opera-

tion, poor circuit board design, and/or high ambient temperature.

A 20°C hysteresis is included so that when thermal shutdown

occurs, the ADP2109 does not return to operation until the

on-chip temperature drops below 130°C. When coming out

of thermal shutdown, soft start is initiated.

SOFT START

The ADP2109 has an internal soft start function that ramps

the output voltage in a controlled manner upon startup, thereby

limiting the inrush current. This prevents possible input voltage

drops when a battery or a high impedance power source is

connected to the input of the converter.

After the EN pin is driven high, internal circuits start to power

up. The time required to settle after the EN pin is driven high is

called the power-up time. After the internal circuits are powered

up, the soft start ramp is initiated and the output capacitor is

charged linearly until the output voltage is in regulation. The

time required for the output voltage to ramp is called the soft

start time.

Start-up time in the ADP2109 is the measure of when the output

is in regulation after the EN pin is driven high. Start-up time

consists of the power-up time and soft start time.

CURRENT LIMIT

The ADP2109 has protection circuitry to limit the amount of

positive current flowing through the PFET switch and through

the synchronous rectifier. The positive current limit on the

power switch limits the amount of current that can flow from

the input to the output. The negative current limit prevents the

inductor current from reversing direction and flowing out of

the load.

100% DUTY OPERATION

reaches the limit where, even with the PFET switch on 100%

this limit, the ADP2109 smoothly transitions to a mode where

the PFET switch stays on 100% of the time. When the input

conditions change again and the required duty cycle falls,

the ADP2109 immediately restarts PWM regulation without