Datasheet

ADP2370/ADP2371 Data Sheet
Rev. C | Page 22 of 32
FEATURES DESCRIPTIONS
PRECISION ENABLE
The enable circuit of the ADP2370/ADP2371 minimizes the
input current during shutdown and simultaneously provides
an accurate enable threshold. When the enable input voltage
is below 400 mV, the regulators are in shutdown mode and the
supply current is typically 1.2 μA. As the enable input voltage
rises above the standby enable threshold of 1.0 V, the internal
bias currents and voltages are activated, turning on the precision
enable circuitry. This allows the precision enable circuitry to
detect accurately when the EN pin voltage exceeds the precision
enable rising threshold of 1.2 V.
FORCED PWM OR PWM/PSM SELECTION
Connecting the SYNC pin to a voltage greater than 1.2 V forces
the device to operate permanently in the PWM mode. This means
that the ADP2370/ADP2371 continue to operate at a fixed fre-
quency even when the output current is less than the PWM/PSM
threshold. In PWM mode, the efficiency is lower compared to the
PSM mode during light loads. The low-side NMOS remains on
when the output current drops to less than zero thereby preventing
the device from entering discontinuous conduction (DCC) mode.
It is possible to switch from FPWM mode to the power-save
mode during operation by pulling the SYNC pin low. The
flexible configuration of the SYNC pin during operation of
the device allows for efficient power management.
Connecting the SYNC pin to a voltage less than 0.4 V allows
the part to operate in either PWM or PSM modes, depending on
the output current. Whenever the average output current goes
below the PWM/PSM threshold, the ADP2370/ADP2371 enter
PSM mode operation. During PSM mode the part operates with
reduced switching frequency and with a minimal quiescent cur-
rent to maintain high efficiency. The low-side NMOS turns off
when the output current reaches zero, causing the part to operate
in DCC mode.
QUICK OUTPUT DISCHARGE (QOD) FUNCTION
The ADP2371 includes an output discharge resistor that forces the
output voltage to zero when the buck is disabled. This ensures
that the output of the buck is always in a well-defined state, whether
or not it is enabled. The ADP2370 does not include this output
discharge function.
SHORT-CIRCUIT PROTECTION
The ADP2370/ADP2371 include frequency foldback to prevent
output current runaway on a hard short. When the voltage at
the feedback pin falls below 0.3 V, indicating the possibility of
a hard short at the output, the switching frequency is reduced
to 1/4 of the internal oscillator frequency. The reduction in the
switching frequency gives more time for the inductor to dis-
charge, preventing a runaway of output current.
UNDERVOLTAGE LOCKOUT
To protect against battery discharge, an undervoltage lockout
(UVLO) circuit is incorporated into the ADP2370/ADP2371.
When the input voltage drops below the UVLO threshold, the
ADP2370/ADP2371 shuts down, and both the power switch
and synchronous rectifier turn off. Once the input voltage rises
above the UVLO threshold, the soft start period is initiated and
the device is enabled.
THERMAL PROTECTION
In the event that the junction temperature on either the ADP2370
or ADP2371 rises above 150°C, the thermal shutdown protec-
tion circuit turns off the regulator. Extreme junction temperature
can be the result of high current operation, poor circuit board
design, and/or high ambient temperature. A 20°C hysteresis is
included in the protection circuit so that when a thermal shut-
down occurs, the device does not return to operation until the
on-chip temperature drops below 130°C. When exiting a
thermal shutdown, soft start is initiated.
SOFT START
The ADP2370/ADP2371 have 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 imped-
ance power source is connected to the input of the converter.
Typical soft start time is 350 μs. The ADP2370/ADP2371 are
also capable of starting up into a precharged output capacitor.
If soft start is invoked when the output capacitor charge is greater
than zero, the device delays the start of switching until the internal
soft start ramp reaches the corresponding FB voltage. This fea-
ture prevents discharging the output capacitor at the beginning
of soft start.
CURRENT LIMIT
The ADP2370/ADP2371 have protection circuitry that limits
the direction and amount of current to 1200 mA that flows
through the power switch and synchronous rectifier, cycle by
cycle. 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 on the synchronous rectifier prevents
the inductor current from reversing direction and flowing out
of the load.
A negative current limit is provided by the ADP2370/ADP2371
to prevent an excessive reverse inductor current when the switching
section sinks current from the load in forced continuous con-
duction mode. Under negative current-limit conditions, both
the high-side and low-side switches are disabled.