RT8205A RICHTEK [Richtek Technology Corporation], RT8205A Datasheet - Page 20

• Manufacturer Part Number: RT8205A
• Description: High Efficiency, Main Power Supply Controllers for Notebook Computers
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8205A/B/C
Operation Mode Selection (SKIPSEL)
The RT8205A/B/C supports three operation modes : Diode-
Emulation Mode, Ultrasonic Mode, and Forced-CCM
Mode.
Diode-Emulation Mode (SKIPSEL = REF)
In Diode-Emulation mode, The RT8205A/B/C automatically
reduces switching frequency at light-load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly and without increase of V
load regulation. As the output current decreases from
heavy-load condition, the inductor current is also reduced,
and eventually comes to the point that 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 of negative current when the inductor
free-wheeling current reach 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 can be calculated as
follows (Figure 1) :
where Ton is the On-time.
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www.richtek.com
20
I
LOAD(SKIP)
VREG5 or
VREG5 or
VREG3
VREG3
GND
GND
TON
TON
Table 1. TONSEL Connection and Switching
REF
REF
©
2012 Richtek Technology Corporation. All rights reserved.
≈
Frequency (kHz)
K-Factor
(V
SMPS 1
IN
3.33
(μs)
2.5
SMPS 2
5
−
2L
250
375
500
V
Frequency
OUT
)
×
Frequency
T
SMPS 1
ON
(kHz)
K-Factor Error (%)
200
300
400
Approximate
±10
±10
±10
K-Factor
OUT
SMPS 2
2.67
(μs)
ripple or
4
2
The switching waveforms may appear noisy and
asynchronous when light loading causes Diode-Emulation
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM noise
vs. light-load efficiency are 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)
Connecting SKIPSEL to VREG5 or VREG3 activates a
unique Diode-Emulation mode with a minimum switching
frequency of 25kHz. This ultrasonic 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 with an internal oscillator (>25kHz).
Once the internal oscillator is triggered, the ultrasonic
controller pulls LGATEx high, turning on the low-side
MOSFET to induce a negative inductor current. After the
output voltage across the REF, the controller turns off the
low-side MOSFET (LGATEx pulled low) and triggers a
constant on-time (UGATEx driven high). When the on-
time has expired, the controller re-enables the low-side
MOSFET until the controller detects that the inductor
current dropped below the zero-crossing threshold.
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 gate-
is a registered trademark of Richtek Technology Corporation.
Figure 1. Boundary condition of CCM/DCM
I
0
L
Slope = (V
t
ON
IN
-V
OUT
) / L
DS8205A/B/C-06 July 2012
i
i
L, peak
Load
t
= i
L, peak
/ 2