RT8205L 查看數據表（PDF） - Richtek Technology

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RT8205L

High Efficiency, Main Power Supply Controller for Notebook Computer

Richtek Technology 

RT8205L/M

TONSEL

GND

REF

VREG5 or

VREG3

Table 1. TONSEL Connection and Switching Frequency

SMPS 1

SMPS 1

SMPS 2

K-Factor (μs) Frequency (kHz) K-Factor (μs)

SMPS 2

Frequency (kHz)

5

200

4

250

3.33

300

2.67

375

2.5

400

2

500

Approximate K-Factor

Error (%)

±10

±10

±10

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) :

IL

Slope = (VIN -VOUT) / L

IL, PEAK

ILoad = IL, PEAK / 2

0

tON

t

Figure 1. Boundary Condition of CCM/DEM

ILOAD(SKIP)

≈

(VIN

− VOUT

2L

)

×

tON

where tON is the On-time.

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

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DS8205L/M-05 June 2011

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