RT8239A/B/C
17
DS8239A/B/C-06 October 2012
www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
+
+
−
OUT
DROP1
ON
IN
DROP1
DROP2
f = (V
V
) / (t
x (V
V
V
))
where V
DROP1
is the sum of the parasitic voltage drops in
the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; V
DROP2
is
the sum of the resistances in the charging path; and t
ON
is the on-time calculated by the RT8239A/B/C.
Operation Mode Selection
The RT8239A/B supports two operation modes : Diode
Emulation Mode and Ultrasonic Mode. The RT8239C only
supports Ultrasonic Mode. The operation mode can be
set via the ENM pin for RT8239A or SECFB pin for
RT8239B.
I
L
t
0
t
ON
Slope = (V
IN
-V
OUT
)/L
I
PEAK
I
LOAD =
I
PEAK
/2
Figure 3. Boundary condition of CCM/DEM
IN
OUT
LOAD(SKIP)
ON
(V
V
)
I
t
2L
−
≈
×
where t
ON
is the on-time.
The switching waveforms may appear noisy and
asynchronous when light loading causes diode emulation
operation. This is normal and results in high efficiency.
Trade offs in PFM 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
The RT8239A/B/C activates a unique type of Diode
Emulation Mode with a minimum switching frequency of
25kHz, called Ultrasonic Mode. This mode eliminates
audio-frequency modulation that would otherwise be
present 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
(>25kHz). Once the internal oscillator is triggered, the
as one or both dead times. It occurs only in PWM mode
when the inductor current reverses at light or negative
load currents. With reversed inductor current, the
inductor's EMF causes PHASEx to go high earlier than
normal, hence extending the on-time by a period equal to
the low to high dead time. For loads above the critical
conduction point, the actual switching frequency is :
Diode Emulation Mode
In Diode Emulation Mode, the RT8239A/B automatically
reduces switching frequency at light load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly. As the output current decreases from
heavy-load condition, the inductor current is also reduced,
and eventually comes to the point that its current valley
touches zero, which is the boundary between continuous
conduction and discontinuous conduction modes. By
emulating the behavior of diodes, the low side MOSFET
allows only partial negative current to flow when the
inductor free wheeling current becomes negative. As the
Part Number
RT8239A
RT8239B
RT8239C
Pin Name
ENM
SECFB
SECFB
Pin-13
Voltage Range
Mode State
4.5V to 5V
ASM
ASM
ASM
2.3V to 3.6V
DEM
DEM
ASM
1.2V to 1.8V
ASM
ASM
ASM
Below 0.8V
Shutdown
UVP
UVP
Table 1. Operation Mode Setting
load current is further decreased, it takes longer and longer
time to discharge the output capacitor to the level that
requires the next
“
ON
”
cycle. The on-time is kept the
same as that in the heavy load condition. In reverse, when
the output current increases from light load to heavy load,
the switching frequency increases to the preset value as
the inductor current reaches the continuous conduction.
The transition load point to the light load operation is shown
in Figure 3. and can be calculated as follows :
