RT6318B datasheet(15/25 Pages) RICHTEK

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RT6318B/C

DS6318B/C-01 December 2021

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is a registered trademark of Richtek Technology Corporation.

Ultrasonic mode is selected by the EN voltage level. When

EN is above 2.3V, it enters normal mode. If EN is in the

range of 0.8V to 1.7V, it enters ultrasonic mode.

On-Time Reduction Function for DEM

In normal diode emulation mode, the output voltage ripple

of converter is proportional to on-time and inversely

proportional to load current. In order to have smaller voltage

ripple in light load application, the RT6318B/C provides a

smart reduction on-time function, which will follow

decreased load current to decrease on-time naturally,

therefore the output voltage ripple can be reduced

effectively.

Linear Regulators (LDO & VCC)

The RT6318B/C includes a 3.3V/5V linear regulators

(LDO). The regulators can supply up to 100mA for external

load, therefore it's recommended to bypass LDO with a

minimum 4.7

μF ceramic capacitor to GND. When VOUT

is higher than the switch over threshold 3.1V (RT6318B)

or 4.7V (RT6318C), an automatic circuit will change the

power source of linear regulator from VIN path to VOUT

path, therefore the power dissipation of linear regulator

will be decrease efficiently.

The RT6318B/C also includes a 5V linear regulator (VCC).

The VCC regulator steps down input voltage to supply

both internal circuitry and gate drivers. Do not connect

the VCC pin to external loads.

Current Limit

The RT6318B/C current limit is fixed 9A and it is a cycle-

by-cycle “valley” type, measuring the inductor current

through the synchronous rectifier during the off-time while

the inductor current ramps down. The current is

determined by measuring the voltage between source and

drain of the synchronous rectifier, adding temperature

compensation for greater accuracy. If the current exceeds

the current limit, the on-time one-shot is inhibited until

the inductor current ramps down below the current limit.

Thus, only when the inductor current is well below the

current limit, another on-time is permitted. If the output

current exceeds the available inductor current (controlled

by the current limit mechanism), the output voltage will

drop. If it drops below the output under-voltage protection

level (see next section) the IC will stop switching to avoid

excessive heat.

Peak Current Limit Protection

The RT6318B/C integrates a high-side MOSFET current

limit protection for preventing inductor saturation or avoiding

any possibility of damage caused by too much inrush

current. For implementing peak current limit protection,

there is a ILIMIT_PEAK level ( 15A) is compared to switching

current during high-side MOSFET turned on, as shown in

Figure 3. As the inductor current IL reaches the ILIMIT_PEAK,

the on-time is terminated to limit the load current. Further,

low-side MODFET is turned on to discharge the inductor

current till VOUT is lower than the internal VREF. For

regulating the output voltage, high-side MOSFET is turned

on which is the cycle-by-cycle current limit. However, if

this situation still exists and the duty cycle is suppressed,

output voltage cannot be regulated and starts falling. Once,

the output voltage below UVP level, the RT6318B/C is

going to be latched.



ILIMIT_PEAK

PWM

IOUT

Figure 3. Peak Current Limitation

Output Over-Voltage Protection and Under-Voltage

Protection

The RT6318B/C includes output over-voltage protection

(OVP). If the output voltage rises above the regulation

level, the high-side switch naturally remains off and the

synchronous rectifier will turn on until the inductor current

reaches the zero or next on-time one-shot is triggered. If

the output voltage exceeds the OVP threshold for longer

than 20

μs (typical), the IC's OVP is triggered. The

RT6318B/C also includes output under-voltage protection

(UVP). If the output voltage drops below the UVP trip

threshold for longer than 20

μs (typical) the IC's UVP is

triggered. The RT6318B/C uses latch-off mode in OVP

and UVP. When the protection function is triggered, the

IC will shut down. The IC stops switching and is latched

off. To restart operation, toggle EN or power the IC off and

then on again.

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