XR76108 Datasheet, PDF (12/17 Pages) Exar Corporation – 8A and 12A Synchronous Step Down COT Regulator

XR76108 and XR76112
Detailed Operation
The XR76108/12 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 turn-on and turn-off. It also selects between two
modes of operation: ‘Forced CCM’ and ‘DCM/CCM’. If EN is
pulled below 1.8V the regulator shuts down. A voltage
between 2.0V and 2.8V selects the Forced CCM mode,
which will run the converter in continuous conduction for
all load currents. A voltage higher than 3.1V 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 2.0V and 2.8V must be applied to
EN/MODE. This can be achieved with an external control
signal that meets the above voltage requirement. The
EN/MODE can be derived from VIN where an external
control is not available. If VIN is well regulated, use a
resistor divider and set the voltage to 2.45V. If VIN varies
over a wide range, the circuit shown in figure 33 can be
used to generate the required voltage.
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
EN/MODE pin and be sequenced with respect to VOUT such
that 2.0≤VEN≤2.8V when VOUT finishes softstart.
If an external 5V control signal is available use a low-pass
RC and set the time constant to RC=5.5×tss where tss is
the softstart time. R should be in the 50k-100k range.
Time t1 can be approximated from t1=2.8×tss. The timing
circuit and diagram are shown in figure 34.
If an external 3.3V control signal is available, use a low-
pass RC and set the time constant to RC=2.1×tss. R
should be in the 50k-100k range. Time t1 can be
approximated from t1=1.9×tss (figure 34).
V
IN
RZ
10k
Zener
MMSZ4685T1G or Equivalent
R3
60.4k, 1%
EN/MODE
R4
71.5k, 1%
Figure 33: Selecting Forced CCM by deriving EN/MODE
from VIN
EXTERNAL CONTROL
R_V
EN/MODE
EN
C
5.0V
Or
3.3V
2.5V
2.0V
DC
V_EN
V_OUT
t1
tss
Figure 34: Timing circuit and diagram for selecting
DCM/CCM by using an external enable signal
© 2013-2014 Exar Corporation
12/17
www.exar.com
Rev. 2A