RT6250A/B
when a lightly loaded controller automatically skips
pulses. In ultrasonic mode, the low-side switch gate driver
signal is “OR”ed with an internal oscillator (>33kHz).
(V V
)
IN
OUT
I
t
ON
LOAD
2L
where tON is the on-time.
Once the internal oscillator is triggered, the controller will
turn on UGATE and give it shorter on-time. When the on-
time expired, LGATE turns on until the inductor current
goes to zero crossing threshold and keep both high-side
and low-side MOSFET off to wait for the next trigger.
Because shorter on-time causes a smaller pulse of the
inductor current, the controller can keep output voltage
and switching frequency simultaneously. The on-time
decreasing has a limitation and the output voltage will be
lifted up under the slight load condition. The controller will
turn on LGATE first to pull down the output voltage. When
the output voltage is pulled down to the balance point of
the output load current, the controller will proceed the
short on-time sequence as the above description.
I
L
Slope = (V - V
) / L
OUT
IN
I
PEAK
ILOAD = I
/ 2
PEAK
t
t
ON
Figure 1. Boundary Condition of CCM/DEM
The switching waveforms may appear noisy and
asynchronous when light load causes diode emulation
operation. This is normal and results in high efficiency.
Trade offs in DEM noise vs. light load efficiency is made
by varying the inductor value.Generally, low inductor values
produce a broader efficiency vs. load curve, while higher
values result in higher full load efficiency (assuming that
the coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values include
larger physical size and degraded load transient response
(especially at low input voltage levels).
Ultrasonic mode is selected by the ENvoltage level. When
ENis above 2.3V, it enters ultrasonic mode. If ENis in the
range of 0.8V to 1.7V, it enters normal mode. If use the
ultrasonic mode must use the RT provide BOM.
On-Time Reduction Function for DEM
In normal diode emulation mode, the output voltage ripple
of converter is proportional to on-time and inversely
proportional to load current. In order to have smaller voltage
ripple in light load application, the RT6250A/B provide a
smart reduction on-time function, which will follow
decreased load current to decrease on-time naturally,
therefore the output voltage ripple can be reduced
effectively.
At boundary condition of discontinuous switching and
continuous, the on-time is immediately increased to add
“hysteresis” to discourage the IC from discontinuous
switching back to continuous switching unless the load
increases substantially. The IC returns to continuous
switching as soon as an on-time is generated before the
inductor current reaches zero. The on-time is reduced back
to the length needed for designed switching frequency
and encouraging the circuit to remain in continuous
conduction, preventing repetitive mode transitions between
continuous switching and discontinuous switching.
Linear Regulators (VCC)
The RT6250A/B also includes a 5V linear regulator (VCC).
The VCC regulator steps down input voltage to supply
both internal circuitry and gate drivers. Do not connect
the VCC pin to external loads.
Bypass Function for VCC
Ultrasonic Mode (USM)
When PGOOD is pulled high and BYP pin of the
RT6250A/B voltage is above 4.7V, an internal 3Ω P-
MOSFET switch connects VCC to the BYP pin while the
VCC linear regulator is simultaneously turned off. Because
The RT6250Aactivates a unique type of diode emulation
mode with a minimum switching period of 30μs (typical),
called ultrasonic mode. This mode eliminates audio-
frequency modulation that would otherwise be present
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS6250A/B-01 May 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
