RT6219
inductor current reaches the continuous conduction. The
transition load point to the light load operation is shown in
Figure 4 and can be calculated as follows :
Linear Regulators (VCC)
The RT6219 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. When PGOOD is pulled high and
BYP pin voltage is above 4.6V, an internal 3Ω
P-MOSFET switch connects VCC to the BYP pin while
the VCC linear regulator is simultaneously turned off.
I
L
Slope = (V - V
) / L
IN
OUT
I
PEAK
ILOAD = I
/ 2
PEAK
Current Limit
The RT6219 current limit is a cycle-by-cycle “valley” type,
measuring the inductor current through the synchronous
rectifier during the off-time while the inductor current ramps
down. The current is determined by measuring the voltage
between Source and Drain of the synchronous rectifier,
adding temperature compensation for greater accuracy. If
the current exceeds the current limit, the on-time one-
shot is inhibited until the inductor current ramps down
below the current limit. If the output current exceeds the
available inductor current (controlled by the current limit
mechanism), the output voltage will drop. If it drops below
the output under-voltage protection level (see next section),
the IC will stop switching to avoid excessive heat.
t
t
ON
Figure 4. Boundary Condition of CCM/DEM
(V V
)
IN
OUT
I
t
ON
LOAD
2L
where tON is the on-time.
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).
Output Over-Voltage Protection and Under-Voltage
Protection
The RT6219 features an output Over-Voltage Protection
(OVP). For the RT6219Aand the RT6219BL, if the output
voltage rises above the regulation level, the IC stops
switching and is latched off. On the other hand, for the
RT6219AH and the RT6219BH, the IC will stop switching
and restart automatically after a short period which is the
so-called Hiccup mode. The RT6219 also features an
output Under-Voltage Protection (UVP). If the output
voltage drops below the UVP trip threshold for longer than
5μs (typical), the UVP is triggered, and the IC will
shutdown. Likewise, for the RT6219Aand the RT6219BL,
the IC stops switching and is latched off. On the other
hand, for the RT6219AH and the RT6219BH, the IC will
stop switching and enter the Hiccup mode. To restart
operation from latch off, toggle ENor power the IC off and
During discontinuous switching, the on-time is immediately
increased to add “hysteresis” to discourage the IC from
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 500kHz switching and encouraging
the circuit to remain in continuous conduction, preventing
repetitive mode transitions between continuous switching
and discontinuous switching.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6219-06 January 2018
www.richtek.com
11
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