RT8205CGQW datasheet(20/28 Pages) RICHTEK

20 / 28 page

RT8205A/B/C

DS8205A/B/C-06 July 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Table 1. TONSEL Connection and Switching

Frequency

TON

SMPS 1

K-Factor

(

μs)

SMPS 1

Frequency

(kHz)

SMPS 2

K-Factor

(

μs)

GND

200

REF

3.33

300

2.67

VREG5 or

VREG3

2.5

400

TON

SMPS 2

Frequency (kHz)

Approximate

K-Factor Error (%)

GND

250

±10

REF

375

±10

VREG5 or

VREG3

500

±10

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)

InDiode-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 VOUT ripple or

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) :

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-

OUT

LOAD(SKIP)

)

−

≈×

where Ton is the On-time.

Figure 1. Boundary condition of CCM/DCM

tON

Slope = (VIN -VOUT) / L

iL, peak

iLoad = iL, peak / 2

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