RT8249 datasheet(16/23 Pages) RICHTEK

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RT8249A/B/C

DS8249A/B/C-02

June 2014

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Figure 1. Boundary Condition of CCM/DEM

OUT

LOAD(SKIP)

)





where tON is the on-time.

The switching waveforms may appear noisy and

asynchronous when light load causes diode emulation

operation. This is normal and results in high efficiency.

tON

Slope = (VIN - VOUT) / L

IPEAK

ILOAD = IPEAK / 2

the SKIPSEL pin voltage is higher than 1.2V, the RT8249C

operates in DEM. When the SKIPSEL pin Voltage is lower

than 0.8V, the RT8249C operates in ASM.

Diode Emulation Mode

In diode emulation mode, the RT8249A/B/C 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. To

emulate 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

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 1. and can be calculated as follows :

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 (ASM)

The RT8249C 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 controller will

turn on UGATE and give it shorter on-time.

When the on-time expired, LGATE turns on until the

inductor current goes to zero crossing threshold and keep

both high-side and low-side MOSFET off to wait for the

next trigger. Because shorter on-time causes a smaller

pulse of the inductor current, the controller can keep output

voltage and switching frequency simultaneously.

The on-time decreasing has a limitation and the output

voltage will be lifted up under the slight load condition.

The controller will turn on LGATE first to pull down the

output voltage. When the output voltage is pulled down to

the balance point of the output load current, the controller

will proceed the short on-time sequence as the above

description.

Linear Regulators (LDOx)

The RT8249A/B/C includes 5V (LDO5) and 3.3V (LDO3)

linear regulators. The regulators can supply up to 100mA

for external loads. Bypass LDOx with 1

μF(min) to 4.7μF

(max), and the recommended value is 1

μF. ceramic

capacitor. When VOUT1 is higher than the switch over

threshold (4.66V), an internal 1.5

Ω P-MOSFET switch

connects BYP1 to the LDO5 pin while simultaneously

disconnects the internal linear regulator.

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