XRP6141 Datasheet, PDF (10/14 Pages) Exar Corporation – 35A Synchronous Step Down COT Controller

XRP6141
35A Synchronous Step Down COT Controller
DETAILED OPERATION
XRP6141 is a synchronous step-down proprietary
emulated current-mode Constant On-Time (COT)
controller. 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 GH signal turning the
high-side (switching) FET 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
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 makes possible the use of
ceramic capacitors, in addition to other capacitor types, for
output filtering.
ENABLE/MODE INPUT (EN/MODE)
EN/MODE pin accepts a tri-level signal that is used to
control turn on/off. It also selects between two modes of
operation: ‘Forced CCM’ and ‘DCM/CCM’. If EN is pulled
below 1.9V, the controller shuts down. A voltage between
1.9V and 3.0V selects the Forced CCM mode which will run
the converter in continuous conduction at all times. A
voltage higher than 3.0V selects the DCM/CCM mode
which will run the converter in discontinuous conduction 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
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 28 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 28 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 4V. If VIN varies over a wide range, the circuit shown in
figure 29 can be used to generate the required voltage.
Figure 28. Selecting Forced CCM by deriving EN/MODE
from VIN
Figure 29. 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:
(3.4𝐸 − 10) × 𝑅𝑂𝑁
𝑇𝑂𝑁 =
𝑉𝐼𝑁
The required TON for a given application is calculated
from:
𝑉𝑂𝑈𝑇
𝑇𝑂𝑁 = _{𝑉𝐼𝑁 × 𝑓}
© 2015 Exar Corporation
10/14
Rev. 2.0.0