XR79106 Datasheet, PDF (11/19 Pages) Exar Corporation – 22V, 6A Synchronous Step-Down

XR79106
Applications Information
Functional Description
XR79106 is a synchronous step-down proprietary
emulated current-mode Constant On-Time (COT) module.
The on-time, which is programmed via R_ON,
is inversely proportional 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 (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 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 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 1.8V,
the module shuts down. A voltage between 2.0V and 2.8V
selects the forced CCM mode which will run the module in
continuous conduction at all times. A voltage higher than
3.1V selects the DCM/CCM mode which will run the module
in discontinuous conduction at light loads.
Selecting the Forced CCM Mode
In order to set the module 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. Where an external
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 25 can be used to generate
the required voltage. Note that at V_IN of 4.5V and 22V
the nominal Zener voltage is 3.8V and 4.7V respectively.
Therefore for V_IN in the range of 4.5V to 22V, the circuit
shown in Figure 25 will generate V_EN required for
forced CCM.
Selecting the DCM/CCM Mode
In order to set the module operation to DCM/CCM, a voltage
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 divider
and set the voltage to 4V. If V_IN varies over a wide range,
the circuit shown in Figure 26 can be used to generate the
required voltage for DCM/CCM operation.
V_IN
RZ
10k
Zener
MMSZ4685T1G or
Equivalent
R1
24.3k, 1%
R2
EN/MODE
35.7k, 1%
Forced CCM, wide VIN range
Figure 25. Selecting Forced CCM by
Deriving EN/MODE from V_IN
V_IN
Zener
MMSZ4685T1G or
Equivalent
RZ
10k
V_EN
EN/MODE
DCM/CCM, wide VIN range
Figure 26. Selecting DCM/CCM by
Deriving EN/MODE from V_IN
REV1B
11/19