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RT8105

5V/12V Synchronous Buck PWM DC/DC Controller

Richtek Technology 

RT8105

Application Information

RT8105 is a voltage-mode single phase synchronous buck

controller with embedded MOSFET drivers. This part

provides complete protection functions such as over voltage

protection, under voltage protection and over current

protection. Inductor current information is sensed by the

RDS(ON) of the low side MOSFET. The over current

protection threshold can be simply programmed by a

resistor. In addition, the compensation circuit is

implemented internally to minimize the external

component count.

VCC Power on Reset and VIN Detection

Once VCC exceeds its power on reset rising threshold

VCCRTH, UGATE will output continuous pulses (~10kHz,

1% duty cycle) for converter input voltage VIN detection.

Figure 1 and figure 2 illustrate the operation of VIN detection

for RT8105. VIN is recognized ready by detecting the

voltage pulses at VOS pin exceed 1.5V for four times

(both rising edge + falling edge for counter increment =

1). Once VIN is recognized ready, controller will initiates

the soft start operation. Since a 40μA current will

continuously flow through ROCSET, ROCSET must be lower

than 37.5kΩ for the correct VIN detection function.

Controller will not initiate soft start if ROCSET is higher than

this value because VIN will not be recognized ready.

3V

IOC

-

OC

OPS

+

10pF

ROCSET

Q2

PHASE

VOC

+

-

Cparasitic

DISABLE

VIN

Detection

Counter

+

UGATE

- 1.5V Internal Counter will count (VOPS > 1.5V) four times

(rising & falling) to recognize VIN is ready.

Figure 1. VIN Detection Function

Soft Start

Once VIN is recognized ready, LGATE will go high for a

short period of time to discharge the pre-biased voltage at

the output capacitor. After that, controller will initiate the

soft start operation. RT8105 provides soft start function

internally. The soft start function is used to prevent the

large inrush current while converter is powered up. The

FB signal will track the internal soft start signal, which is

controlled by an internal digital counter and ramps up from

zero in a monotone during soft start period. Therefore the

duty cycle of UGATE signal will increase gradually and

so does the input current. The typical soft-start duration

is 3ms.

Over Current Protection (OCP)

Figure 2 shows the over current protection (OCP) scheme

of RT8105. A resistor ROCSET connected from PHASE pin

to OPS pin sets the threshold. An internal current source,

IOC (40μA typically), flowing through ROCSET determines

the OCP trip point IOCSET, which can be calculated using

the following Equation :

IOCSET ≈

40uA × ROCSET − 0.4

RDS(ON) of the low side MOSFET

Because the RDS(ON) of MOSFET increases with

temperature, it is necessary to take this thermal effect

into consideration in calculating OCP point.

OC

Comparator

3V

IOC

-

+

+

0.4V

VIN

OPS

ROCSET

PHASE

Q1

+

IOC x ROCSET

-

Q2

L

-

ID x RDS(ON)

+

Figure 2. Over Current Protection Scheme

In addition, note that the OCP threshold is very sensitive

to the parasitic capacitance at OPS pin. Parasitic

capacitance or the drain-to-source capacitance of the small

MOSFET (for shutdown function) will have influence on

the OCP threshold. It is recommended to use small signal

BJT for shutdown function. In addition, it is also

recommended to place ROCSET close to IC to minimize

the trace parasitic.

When OCP is tripped, both UGATE and LGATE will go

low to stop the energy transfer to the load. Controller will

www.richtek.com

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DS8105-03 April 2011

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