RT6243A/B
Table 1. MODE Setting
Mode
1
2
3
4
5
6
7
8
9
10
11
12
R
M1
(k)
300
200
160
120
200
180
150
120
91
82
62
51
R
M2
(k)
5.1
10
20
20
51
51
51
51
51
51
51
51
Light Load Mode
FCCM
FCCM
FCCM
FCCM
FCCM
FCCM
DCM
DCM
DCM
DCM
DCM
DCM
Current Limit
I
LIM_2
I
LIM_1
I
LIM_2
I
LIM_1
I
LIM_2
I
LIM_1
I
LIM_2
I
LIM_1
I
LIM_2
I
LIM_1
I
LIM_2
I
LIM_1
Switching Frequency (kHz)
400
400
800
800
1200
1200
400
400
800
800
1200
1200
Light Load Operation
At low load current, the inductor current can drop to zero
and become negative. This is detected by internal zero-
current-detect circuitry which utilizing the LSFET R
DS(ON)
to sense the inductor current. The LSFET is turned off
when the inductor current drops to zero, resulting in
discontinuous operation (DCM). Both power MOSFETs
will remain off with the output capacitor supplying the load
current until the feedback voltage falls below the feedback
reference voltage. DCM operation maintains high efficiency
at light load, while setting MODE to Forced PWM (FCCM)
operation helps meet tight voltage regulation accuracy
requirements.
Switching Frequency, Minimum On-Time and
Minimum Off-Time
The RT6243A/B offers three different switching frequency
of 400kHz, 800kHz and 1200kHz by setting the MODE
pin voltage. Selection of the operating frequency is a trade-
off between efficiency and component size. High frequency
operation allows the use of smaller inductor and capacitor
values. Operation at lower frequencies improves efficiency
by reducing internal gate charge and transition losses,
but requires larger inductance values and/or capacitance
to maintain low output ripple voltage.
An additional constraint on operating frequency is the
minimum controllable on-time and off-time. The minimum
Copyright
©
2019 Richtek Technology Corporation. All rights reserved.
on-time is the smallest duration of time in which the high-
side power MOSFET (HSFET) can be in its
“on”
state.
This time is typically 54ns. In continuous mode operation,
the minimum duty cycle can be estimated by ignoring
component losses as follows
D
MIN
= f
SW
t
ON_MIN
Where t
ON_MIN
is the minimum on-time. As the equation
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
The minimum off-time, t
OFF_MIN
, is the smallest amount of
time that the RT6243A/B is capable of turning on the low-
side power MOSFET (LSFET), tripping the current
comparator and turning the power MOSFET back off. This
time is 310ns (max.). The minimum off-time limit imposes
a maximum duty cycle of t
ON
/( t
ON
+ t
OFF_MIN
).
Current Limit and Output Under-Voltage Protection
As shown in Table 1, the RT6243A/B can operate at two
different current limits I
LIM_1
and I
LIM_2
to support an output
continuous current of 12A and 10A respectively. The device
cycle-by-cycle compares the valley current of the inductor
against the current limit threshold, hence the output
current will be half the ripple current higher than the valley
current.
The inductor current level is monitored by measuring the
low-side MOSFET voltage between the SW pin and GND,
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
6
DS6243A/B-01 June 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
