RT6318B datasheet(14/25 Pages) RICHTEK

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RT6318B/C

DS6318B/C-01 December 2021

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

High Voltage Conversion Ratio Function

Due to minimum off time limitation, the voltage conversion

ratio will be limited in 2S battery application. Therefore

the RT6318C provides increasing on-time function to

enhance voltage conversion ratio for 2S battery application.

Diode Emulation Mode (DEM)

In diode emulation mode, the RT6318B/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. 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 2. and can be calculated as follows :

OUT

LOAD

)





where tON is the on-time.

Slope = (VIN - VOUT) / L

IPEAK

ILOAD = IPEAK / 2

tON

Figure 2. Boundary Condition of CCM/DEM

The switching waveforms may appear noisy and

asynchronous when light load causes diode emulation

operation. This is normal and results in high efficiency.

Trade offs in DEM 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).

At boundary condition of discontinuous switching and

continuous, the on-time is immediately increased to add

“hysteresis” to discourage the IC from discontinuous

switching back to continuous switching unless the load

increases substantially. The IC returns to continuous

switching as soon as an on-time is generated before the

inductor current reaches zero. The on-time is reduced back

to the length needed for designed switching frequency

and encouraging the circuit to remain in continuous

conduction, preventing repetitive mode transitions between

continuous switching and discontinuous switching.

Ultrasonic Mode (USM)

The RT6318B/C activates a unique type of diode emulation

mode with a minimum switching period of 30

μs (typical),

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 (>33kHz).

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. After decreased

on-time, the controller employs longer LGATE to pull down

the output voltage, which can keep output voltage at correct

threshold.

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