RT8205L RICHTEK [Richtek Technology Corporation], RT8205L Datasheet - Page 18

• Manufacturer Part Number: RT8205L
• Description: High Efficiency, Main Power Supply Controller for Notebook Computer
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8205L/M
Operation Mode Selection (SKIPSEL)
The RT8205L/M supports three operation modes: Diode-
Emulation Mode, Ultrasonic Mode, and Forced-CCM
Mode. User can set operation mode via the SKIPSEL pin.
Diode-Emulation Mode (SKIPSEL = REF)
In Diode-Emulation Mode, the RT8205L/M 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 when its valley touches
zero current, 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 when the inductor free
wheeling current becomes negative. As the load current
is further decreased, it takes longer and longer 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 as follows (Figure 1) :
www.richtek.com
18
VREG5 or
TONSEL
VREG3
GND
REF
I
0
L
Figure 1. Boundary Condition of CCM/DEM
Slope = (V
t
ON
K-Factor (μs)
SMPS 1
IN
3.33
2.5
-V
5
OUT
) / L
Table 1. TONSEL Connection and Switching Frequency
Frequency (kHz)
I
L, PEAK
I
Load
SMPS 1
t
200
300
400
= I
L, PEAK
/ 2
K-Factor (μs)
SMPS 2
2.67
4
2
where t
The switching waveforms may appear noisy and
asynchronous when light loading causes Diode-Emulation
Mode operation. However, 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 (SKIPSEL = VREG5 or VREG3)
The RT8205L/M activates an unique Diode-Emulation Mode
with a minimum switching frequency of 25kHz, called the
Ultrasonic Mode. The Ultrasonic Mode avoids audio-
frequency modulation that would otherwise be present
when a lightly loaded controller automatically skips
pulses. In Ultrasonic Mode, the high side switch gate driver
signal is ORed with an internal oscillator (>25kHz). Once
the internal oscillator is triggered, the controller enters
constant off-time control. When output voltage reaches
the setting peak threshold, the controller turns on the low
side MOSFET until the controller detects that the inductor
current has dropped below the zero crossing threshold.
The internal circuitry provides a constant off-time control,
and it is effective to regulate the output voltage under light
load condition.
Forced CCM Mode (SKIPSEL = GND)
The low noise, Forced CCM mode (SKIPSEL = GND)
disables the zero crossing comparator, which controls
the low side switch on-time. This causes the low side
I
LOAD(SKIP)
ON
is the On-time.
Frequency (kHz)
≈
(V
SMPS 2
IN
250
375
500
−
2L
V
OUT
)
×
t
ON
DS8205L/M-05 June 2011
Approximate K-Factor
Error (%)
±10
±10
±10