XR75100 Datasheet, PDF (10/15 Pages) Exar Corporation – 40V Synchronous Step Down COT Controller

XR75100
Functional Description
XR75100 is a synchronous step-down proprietary emulated
current-mode Constant On-Time (COT) controller. The on-
time, which is programmed via R_ON, is inversely propor-
tional to V_IN and maintains a nearly constant frequency.
The emulated current-mode control is stable with ceramic
output capacitors.
Each switching cycle begins with GH signal turning on the
high-side (control) 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 mini-
mum time (250ns nominal). This parameter is termed Mini-
mum 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 V_FB 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 fil-
tering.
Enable/Mode Input (EN/MODE)
EN/MODE pin accepts a tri-level signal that is used to con-
trol turn on/off. It also selects between two modes of opera-
tion: ‘Forced CCM’ and ‘DCM/CCM.’ If EN is pulled below
1.8V, the controller shuts down. A voltage between 2.0V
and 2.9V selects the Forced CCM mode which will run the
converter in continuous conduction at all times. A voltage
higher than 3.1V selects the DCM/CCM mode which will
run the converter in discontinuous conduction at light loads.
divider and set the voltage to 4V. If V_IN varies over a wide
range, the circuit shown in figure 19 can be used to gener-
ate the required voltage.
V
IN
RZ
10k
Z ener
M M SZ4685T1G or Equivalent
R1
30.1k, 1% _{EN /M OD E}
R2
35.7k, 1%
Figure 18: Selecting Forced CCM by deriving EN/MODE from
VIN
V
IN
Zener
MMSZ4685T1G or Equivalent
RZ
10k
V
EN
EN/MODE
Selecting the Forced CCM Mode
In order to set the controller to operate in Forced CCM, a
voltage between 2.0V and 2.9V must be applied to EN/
MODE. This can be achieved with an external control signal
that meets the above voltage requirement. Where an exter-
nal control is not available, the EN/MODE can be derived
from V_IN. If V_IN is well regulated, use a resistor divider and
set the voltage to 2.5V. If V_IN varies over a wide range, the
circuit shown in figure 18 can be used to generate the
required voltage. Note that at V_IN of 5.5V and 40V the nom-
inal Zener voltage is 4.0V and 5.0V respectively. Therefore
for V_IN in the range of 5.5V to 40V, the circuit shown in fig-
ure 18 will generate V_EN required for Forced CCM.
Selecting the DCM/CCM Mode
In order to set the controller operation to DCM/CCM, a volt-
age between 3.1V and 5.5V must be applied to EN/MODE
pin. If an external control signal is available, it can be
directly connected to EN/MODE. In applications where an
external control is not available, EN/MODE input can be
derived from V_IN. If V_IN is well regulated, use a resistor
Figure 19: Selecting DCM/CCM by deriving EN/MODE from
VIN
DCM Operation
When DCM operation is enabled, the Zero Cross Detect
comparator in the XR75100 senses when the current in the
inductor reaches 0Amps and turns off the low side MOS-
FET. The low side MOSFET is operated to emulate the
operation of a diode preventing the inductor current from
flowing in the negative direction. In this mode the device is
now operating in Pulse Frequency Modulation (PFM) con-
trol. As the load reduces the frequency reduces and thus
the switching losses are reduced resulting in much better
efficiency at light load. The Zero Cross comparator monitors
the voltage across the low side MOSFET to determine the
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