ADP2121

Rev. B | Page 16 of 20

delays in the current-limit comparator allow a slight increase and

variation in this specification.

Soft Start

To prevent excessive input inrush current at startup, the ADP2121

operates with an internal soft start. When EN goes high, or when

the part recovers from a fault (UVLO, TSD, or short-circuit

protection), a soft start timer begins. The soft start timer cor-

responds to the maximum soft start period for the given fixed

output voltage. During this time, the peak current limit is gradually

increased to its maximum. As seen in Figure 40 through Figure 45,

the output voltage passes through several stages to ensure that the

converter is able to start up effectively and in proper sequence.

After the soft start period has expired, the peak current limit

remains at 1 A (typical), and the part enters the operating mode

determined by the MODE pin.

Synchronous Rectification

In addition to the P-channel MOSFET switch, the ADP2121

includes an N-channel MOSFET switch to build the synchronous

rectifier. The synchronous rectifier improves efficiency, especially

for small load currents, and reduces cost and board space by

eliminating the need for an external rectifier.

Compensation

The control loop is internally compensated to deliver maxi-

mum performance with no additional external components.

The ADP2121 is designed to work with 0.47 μH chip inductors

and 4.7 μF capacitors (see Table 6, Table 7, and Table 8.) Other

values may reduce performance and/or stability.

Undervoltage Lockout (UVLO)

If the input voltage is below the UVLO threshold, the ADP2121

automatically turns off the power switches and places the part

into a low power consumption mode. This prevents potentially

erratic operation at low input voltages. The UVLO levels have

approximately 100 mV of hysteresis to ensure glitch-free startup.

Output Short-Circuit Protection

If the output voltage is inadvertently shorted to GND, a standard

dc-to-dc controller delivers maximum power into that short.

This may result in a potentially catastrophic failure. To prevent

this, the ADP2121 senses when the output voltage is below the

short-circuit protection threshold (typically 1.24 V).

At this point, the controller turns off for approximately 1.8 ms

sequence. This cycle repeats until the short is removed or the

part is disabled. This dramatically reduces the power delivered

into the short circuit, yet still allows the converter to recover if

the fault is removed.

TIME (1ms/DIV)

OUTPUT VOLTAGE (200mV/DIV)

INDUCTOR CURRENT (500mA/DIV)

Thermal Shutdown (TSD) Protection

The ADP2121 also includes TSD protection. If the die

temperature exceeds 150°C (typical), the TSD protection

activates and turns off the power devices. They remain off

until the die temperature falls below 135°C (typical), at

which point the converter restarts.