XR76116 Datasheet, PDF (13/19 Pages) Exar Corporation – PowerBloxTM 15A and 20A Synchronous

XR76116/20
Applications Information
Detailed Operation
The XR76116/20 uses a synchronous step-down proprietary
emulated current-mode Constant On-Time (COT) control
scheme. 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 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 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 allows for the use of ceramic capacitors,
in addition to other capacitor types, for output filtering.
Enable
The enable input provides precise control for startup.
Where bus voltage is well regulated, the enable input
can be derived from this voltage with a suitable resistor
divider. This ensures that XR76116/20 does not turn on
until bus voltage reaches the desired level. Therefore the
enable feature allows implementation of undervoltage
Lockout for the bus voltage PV_IN. Simple sequencing can
be implemented by using the PGOOD signal as the enable
input of a succeeding XR76116/20. Sequencing can also
be achieved by using an external signal to control the
enable pin.
Selecting the Forced CCM Mode
A voltage higher than 2.4V at the FCCM pin forces the
XR76116/20 to operate in continuous conduction mode
(CCM). Note that discontinuous conduction mode (DCM)
is always on during soft-start. DCM will persist following
soft-start until a sufficient load is applied to transition
the regulator to CCM. Magnitude of the load required to
transition to CCM is ΔI_L/2, where ΔI_L is peak-to-peak inductor
current ripple. Once the regulator transitions to CCM it will
continue operating in CCM regardless of the load magnitude.
Selecting the DCM/CCM Mode
The DCM will always be available if a voltage less
than 0.4V is applied to the FCCM pin. XR76116/20
will operate in either DCM or CCM depending on the
load magnitude. At light loads DCM significantly increases
efficiency as seen in Figures 3-6. A preload of 10mA is
recommended for DCM operation. This helps improve
voltage regulation when external load is less then 10mA
and may reduce voltage ripple.
Programming the On-Time
The on-time t_ON is programmed via resistor R_ON according
to following equation:
VIN × [tON – (2.5 × 10^-8)]
RON =
3.45 × 10^-10
A graph of t_ON versus R_ON, using the above equation,
is compared to typical test data in Figure 23. The graph
shows that calculated data matches typical test data
within 3%.
The t_ON corresponding to a particular set of operating
conditions can be calculated based on empirical data from:
VOUT
tON = _{VIN x 1.06 x f x Eff.}
Where:
■ f is the desired switching frequency at
nominal I_OUT.
■ Eff. is the converter efficiency corresponding to
nominal I_OUT.
Substituting for t_ON in the first equation we get:
RON =
VOUT
1.06 x f x Eff.
– [(2.5 × 10^-8) x VIN]
(3.45 × 10^-10)
Now R_ON can be calculated in terms of operating
conditions V_IN, V_OUT, f and efficiency using the
above equation.
At V_IN = 12V, f = 800kHz, I_OUT = 20A and using the
efficiency numbers from Figure 3 we get the following R_ON
for XR76120:
V_OUT (V)
5.0
3.3
2.5
1.8
1.5
1.2
1.0
Eff. (%)
0.95
0.93
0.91
0.89
0.87
0.84
0.81
f (kHz)
600
600
800
800
800
800
800
R_ON (kΩ)
23.12
15.30
8.52
6.04
5.02
4.01
3.35
XR76120 R_ON for common output voltages,
V_IN = 12V, I_OUT = 20A
REV1A
13/19