XR79103EL-F Datasheet, PDF (11/20 Pages) Exar Corporation – 22V, 3A Synchronous Step-Down COT Power Module

XR79103
Applications Information
Functional Description
The XR79103 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 pre-programmed 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)
The 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