RT8063 datasheet(8/11 Pages) RICHTEK

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RT8063

DS8063-07

November 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The basic IC application circuit is shown in Typical

Application Circuit. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

followed by CIN and COUT.

Main Control Loop

During normal operation, the internal high side power

switch (P-MOSFET) is turned on at the beginning of each

clock cycle. The inductor current increases until it reaches

the value defined by the output voltage (VCOMP) of the error

amplifier. The error amplifier adjusts its output voltage by

comparing the feedback signal from a resistive voltage

divider on the FB pin with an internal 0.8V reference. When

the load current increases, it causes a reduction in the

feedback voltage relative to the reference. The error amplifier

increases its output voltage until the average inductor

current matches the new load current. When the high

side power MOSFET shuts off, the synchronous power

switch (N-MOSFET) turns on until the beginning of the

next clock cycle.

Output Voltage Setting

The output voltage is set by an external resistive voltage

divider according to the following equation :

where VREF is 0.8V typical. The resistive voltage divider

allows the FB pin to sense a fraction of the output voltage

as shown in Figure 1.

RT8063

GND

VOUT

Figure 1. Setting the Output Voltage

Soft-Start

The RT8063 includes an internal soft-start function that

gradually raises the clamp on the COMP pin.

OUT

REF

= V

1 +

⎛⎞

×⎜⎟

⎝⎠

Operating Frequency

Selection of the operating frequency is a tradeoff between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values,

but at the expense of efficiency. On the other hand,

operation at lower frequency improves efficiency by

reducing internal gate charge and switching losses, but

requires larger inductance and/or capacitance to maintain

low output ripple voltage.

The operating frequency of the IC is determined by an

external resistor, ROSC, that is connected between the RT

pin and ground. The value of the resistor sets the ramp

current that is used to charge and discharge an internal

timing capacitor within the oscillator. The practical switching

frequency ranges from 200kHz to 2MHz. However, when

the RT pin is floating, the internal frequency is set at 2MHz.

Determine the RT resistor value by examining the curve

below. Please notice the minimum on time is about 90ns.

Figure 2. Switching Frequency vs. RRT

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current, DIL, increases with higher VIN and decreases

with higher inductance :

OUT

I =

1 -

f x L

⎡⎤ ⎛

⎞

Δ×⎜⎟

⎢⎥

⎣⎦ ⎝

⎠

0.0

0.4

0.8

1.2

1.6

2.0

2.4

300

600

900

1200

1500

1800

2100

RRT (k )

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