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RT8120B

Single-Phase Synchronous Buck PWM Controller

Richtek Technology 

RT8120

Application Information

Function Description

The RT8120 is a single-phase synchronous buck PWM

controller with integrated N-MOSFET gate drivers. The

RT8120 can be used in a broad variety of applications,

with its wide input voltage range from 3V or 13.2V. It

provides single feedback loop, voltage mode control with

fast transient response. An internal 0.8V (option for 0.6V)

reference allows the output voltage to be precisely

regulated for low output voltage applications. A fixed

frequency (300kHz) oscillator is integrated to minimize

external components. Protection features include

programmable over current protection and Under Voltage

Lockout (UVLO).

Supply Voltage and Power On Reset (POR)

The input voltage range for VCC is from 4.5 V to 13.2 V

with respect to GND. An internal linear regulator regulates

the supply voltage for internal control logic circuit. A

minimum 0.1μF ceramic capacitor is recommended to

bypass the supply voltage. Place the bypassing capacitor

physically near the IC. VCC also supplies the integrated

MOSFET drivers. A bootstrap diode is embedded to

facilitate PCB design and reduce the total BOM cost. No

external Schottky diode is required in real applications.

The Power-On Reset (POR) circuit monitors the supply

voltage at the VCC pin. If VCC exceeds the POR rising

threshold voltage (typ. 4V), the controller resets and

prepares the PWM for operation. If VCC falls below the

POR falling threshold during normal operation, all

MOSFETs stop switching. The POR rising and falling

threshold has a hysteresis (typ.0.45V) to prevent

unintentional noise based reset.

Chip Enable and Disable

The COMP/EN pin of the RT8120 is a multiplexed pin.

During soft-start and normal converter operation, this pin

represents the output of the error amplifier. When COMP/

EN pin voltage falls or is pulled externally below the enable

level VEN, the chip shuts down. When the controller shuts

down, UGATE and LGATE signals will go low. When the

pull-down device is released and the COMP/EN pin rises

above the VEN trip point, the RT8120 will begin a new

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initialization and soft-start cycle. This allows flexible power

sequence control for specified application. In practical

applications, connect a small-signal MOSFET to the

COMP/EN pin to implement the enable/disable function.

VIN Detection (RT8120C/D Only)

Once VCC exceeds its power on reset (POR) rising

threshold voltage, UGATE will output continuous pulses

(~60kHz, 200ns), and LGATE will be forced low for

converter input voltage VIN detection. If the voltage pulses

at the PHASE pin exceed 1V when UGATE is turned on,

VIN is recognized as ready. Then, the controller will initiate

soft-start operation.

Internal Soft-Start

The RT8120 provides an internal soft-start function. The

soft-start function is used to prevent large inrush current

and output voltage overshoot while the converter is being

powered-up. The soft-start function automatically begins

once the chip is enabled. An internal current source

charges the internal soft-start capacitor such that the

internal soft-start voltage ramps up uniformly. The FB

voltage will track the internal soft-start voltage during the

soft-start interval. Therefore, the PWM pulse width

increases gradually to limit the input current. After the

internal soft-start voltage exceeds the reference voltage,

the FB voltage no longer tracks the soft-start voltage but

rather follows the reference voltage. Therefore, the duty

cycle of the UGATE signal as well as the input current at

power up are limited.

Over Current Protection (OCP)

The RT8120 provides lossless over current protection by

detecting the voltage drop across the low side MOSFET

when it is turned on. The over current trip threshold is set

by an external resistor, ROCSET, at LGATE. During the initial

stage when LGATE is turned on, the RT8120 samples

and holds the phase voltage. The sample-and-hold voltage

represents the valley inductor current and is compared to

the OCP threshold. If the sensed phase voltage is lower

than the OCP threshold, OCP will be triggered. Both

UGATE and LGATE will go low, and the controller will enter

the hiccup mode until the OCP condition is released.

is a registered trademark of Richtek Technology Corporation.

DS8120-08 September 2013

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