RT8120 datasheet(11/17 Pages) RICHTEK

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RT8120

DS8120-08

September 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

LGATE Over Current Setting (LGOCS)

Over current threshold is externally programmed by adding

a resistor (ROCSET) between LGATE and GND. Once VCC

exceeds the POR threshold, an internal current source

IOCSET flows through ROCSET. The voltage across ROCSET is

stored as the over current protection threshold VOCSET.

After that, the current source is switched off. ROCSET can

be determined using the following equation :

VALLEY

LGDS(ON)

OCSET

x R

where IVALLEY represents the desired inductor OCP trip

current (valley inductor current).

If ROCSET is not present, there is no current path for IOCSET

to build the OCP threshold. In this situation, the OCP

threshold is internally preset to 375mV (typical).

Under Voltage Protection (UVP)

The voltage on the FB pin is monitored for under voltage

protection. If the FB voltage is lower than the UVP threshold

(typically 75% x VREF) during normal operation, UVP will

be triggered. When the UVP is triggered, both UGATE

and LGATE go low. The controller enters hiccup mode

until the UVP condition is removed.

Output Voltage Setting

The RT8120 allows the output voltage of the DC/DC

converter to be adjusted from 0.8V (option for 0.6V) to

85% of VIN via an external resistor divider. It will try to

maintain the feedback pin at internal reference voltage

(0.8V, with option for 0.6V).

Figure 3. Output Voltage Setting

According to the resistor divider network above, the output

voltage is set as :

FB1

OUT

REF

FB2

= V

x 1 +

⎛⎞

⎜⎟

⎝⎠

MOSFET Drivers

The RT8120 integrates high current gate drivers for the

two N-MOSFETs to obtain high efficiency power conversion

in synchronous buck topology. A dead time is used to

prevent crossover conduction for the high side and low

side MOSFETs. Because both gate signals are off during

dead time, the inductor current freewheels through the

body diode of the low side MOSFET. The freewheeling

current and the forward voltage of the body diode contribute

to power loss. The RT8120 employs constant dead time

control scheme to ensure safe operation without

sacrificing efficiency. Furthermore, elaborate logic circuit

is implemented to prevent cross conduction.

For high output current applications, two or more power

MOSFETs are usually paralleled to reduce RDS(ON). The

gate driver needs to provide more current to switch on/off

these paralleled MOSFETs. Gate driver with lower source/

sink current capability result in longer rising/ falling time

in gate signals, and therefore higher switching loss.

The RT8120 embeds high current gate drivers to obtain

high efficiency power conversion. The embedded drivers

contribute to the majority of the power dissipation of the

controller. Therefore, SOP package is chosen for its power

dissipation rating. If no gate resistor is used, the power

dissipation of the controller can be approximately

calculated using the following equation :

VOUT

RFB2

RFB1

DRIVER

BOOT

G_LOW SIDE

DRIVER_LOW SIDE

= f

x (Q

x V

)

where VBOOT represents the voltage across the bootstrap

capacitor and fSW is the switching frequency.

It is important to ensure the package can dissipate the

switching loss and have enough room for safe operation.

Inductor Selection

The inductor plays an importance role in step-down

converters because it stores the energy from the input

power rail and then releases the energy to the load. From

the viewpoint of efficiency, the dc resistance (DCR) of the

inductor should be as small as possible to minimize the

conduction loss. In addition, the inductor covers a

significant proportion of the board space, so its size is

also important. Low profile inductors can save board space

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