XR79120 Datasheet, PDF (12/19 Pages) Exar Corporation – 22V, 20A Synchronous Step Down COT Power Module

XR79120
Functional Description
XR79120 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 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 (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 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 Module shuts down. A voltage between 2.0V and
2.8V selects the Forced CCM mode which will run the Mod-
ule 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.
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 gener-
ate the required voltage.
V
IN
RZ
10k
Zener
MMSZ4685T1G or Equivalent
R1
30.1k, 1% _EN/MODE
R2
35.7k, 1%
Figure 25: Selecting Forced CCM by deriving EN/MODE from
V_IN
Selecting the Forced CCM Mode
In order to set the Module to operate in Forced CCM, a volt-
age 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 cir-
cuit shown in figure 25 can be used to generate the
required voltage. Note that at V_IN of 4.5V and 22V the nom-
inal Zever voltage is 3.8V and 4.7V respectively. Therefore
for V_IN in the range of 4.5V to 22V, the circuit shown in fig-
ure 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 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
V
IN
Zener
MMSZ4685T1G or Equivalent
RZ
10k
V
EN
EN/MODE
Figure 26: Selecting DCM/CCM by deriving EN/MODE from
V_IN
© 2014 Exar Corporation
12 / 19
exar.com/XR79120
Rev 1A