ADP2109 Analog Devices, ADP2109 Datasheet - Page 11

• Manufacturer Part Number: ADP2109
• Description: Compact 600 mA, 3 MHz, Step-Down Converter with Output Discharge
• Manufacturer: Analog Devices

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.
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.
Figure 27. Internal Discharge Switch on Feedback
FB
Rev. A | Page 11 of 16
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
With a drop in V
reaches the limit where, even with the PFET switch on 100%
of the time, V
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
allowing overshoot on V
OUT
IN
drops below the desired output voltage. At
, or an increase in I
OUT
.
LOAD
, the ADP2109
ADP2109