XR76115 Datasheet, PDF (11/16 Pages) Exar Corporation – 15A Synchronous Step Down COT Regulator

XR76115
Detailed Operation
The XR76115 uses a synchronous step-down proprietary
emulated current-mode Constant On-Time (COT) control
scheme. The on-time, which is programmed via RON, is
inversely proportional to VIN and maintains a nearly
constant frequency. The emulated current-mode control
allows the use of ceramic output capacitors.
Each switching cycle begins with the high-side (switching)
FET turning on for a preprogrammed time. At the end of
the on-time, the high-side FET is turned off and the low-
side (synchronous) FET is turned on for a preset minimum
time (250ns nominal). This parameter is termed the
Minimum Off-Time. After the minimum off-time the voltage
at the feedback pin FB is compared to an internal voltage
ramp at the feedback comparator. When VFB drops below
the ramp voltage, the high-side FET is turned on and the
cycle repeats. This voltage ramp constitutes an emulated
current ramp and allows for the use of ceramic capacitors,
in addition to other capacitor types, for output filtering.
Enable/Mode
The EN/MODE pin accepts a tri-level signal that is used to
control channel turn-on and turn-off. It also selects
between two modes of operation: ‘Forced CCM’ and
‘DCM/CCM’. If EN is pulled below 1.9V the regulator shuts
down. A voltage between 1.9V and 3V selects the Forced
CCM mode, which will run the converter in continuous
conduction for all load currents. A voltage higher than 3V
selects the DCM/CCM mode, which will run the converter
in discontinuous conduction mode at light loads.
Selecting the Forced CCM Mode
In order to set the controller to operate in Forced CCM, a
voltage between 1.9V and 3.0V must be applied to the
EN/MODE pin. This can be achieved with an external
control signal that meets the above voltage requirement.
Where an external control is not available, the EN/MODE
signal can be derived from VIN. If VIN is well regulated, use
a resistor divider and set the voltage to 2.5V. If VIN varies
over a wide range, the circuit shown in Figure 27 can be
used to generate the required voltage. Note that at VIN of
5.5V to 22V, the nominal Zener voltage is 4.0V to 5.0V
respectively. Therefore, for VIN in the range of 5.5V to 22V,
the circuit shown in Figure 27 will generate voltage at the
EN/MODE pin required for Forced CCM.
Selecting the DCM/CCM Mode
In order to set the controller operation to DCM/CCM, a
voltage between 3.1V and 5.5V must be applied to the
EN/MODE pin. If an external control signal is available, it
can be directly connected to the EN/MODE pin. In
applications where an external control signal is not
available, EN/MODE input can be derived from VIN. If VIN is
well regulated, use a resistor divider and set the voltage to
4.0V. If VIN varies over a wide range, the circuit shown in
Figure 28 can be used to generate the required voltage.
Figure 27: Selecting Forced CCM by deriving EN/MODE
from VIN
Figure 28: Selecting DCM/CCM by deriving EN/MODE
from VIN
Programming the On-Time
The on-time TON is programmed via resistor RON according
to following equation:
2.5 10
3 10
TON is calculated from:
.
Where:
f is the desired switching frequency at nominal IOUT
© 2015 Exar Corporation
11/16
www.exar.com
Rev. 1B