RT9277APF datasheet(10/12 Pages) RICHTEK

10 / 12 page

RT9277A/B

Preliminary

DS9277A/B-02 March 2007

www.richtek.com

Application Information

The design procedure of Boost converter can start from

the maximum input current, which is related about inductor,

catch-diode input/output capacitor selections and the

maximum power which internal switch can stand. It can

be derived from maximum output power, minimum input

voltage and the efficiency of Boost converter. Once the

maximum input current is calculated, the inductor value

can be determined and the other components as well.

Inductor Selection

For a better efficiency in high switching frequency converter,

the inductor selection has to use a proper core material

such as ferrite core to reduce the core loss and choose

low ESR wire to reduce copper loss. The most important

point is to prevent the core saturated when handling the

maximum peak current. Using a shielded inductor can

minimize radiated noise in sensitive applications. The

maximum peak inductor current is the maximum input

current plus the half of inductor ripple current. The

calculated peak current has to be smaller than the current

limitation in the electrical characteristics. A typical setting

of the inductor ripple current is 20% to 40% of the

maximum input current. If the selection is 40%, the

maximum peak inductor current is :

The minimum inductance value is derived from the following

equation :

Depending on the application, the recommended inductor

value is between 2.2

μH to 10μH.

Diode Selection

To achieve high efficiency, Schottky diode is good choice

for low forward drop voltage and fast switching time. The

output diode rating should be able to handle the maximum

output voltage, average power dissipation and the pulsating

diode peak current.

Input Capacitor Selection

For better input bypassing, low-ESR ceramic capacitors

are recommended for performance. A 10

μF input capacitor

is sufficient for most applications. For a lower output power

requirement application, this value can be decreased.

Output Capacitor Selection

For lower output voltage ripple, low-ESR ceramic capacitors

are recommended. The tantalum capacitors can be used

as well, but the ESR is bigger than ceramic capacitor. The

output voltage ripple consists of two components: one is

the pulsating output ripple current flows through the ESR,

and the other is the capacitive ripple caused by charging

and discharging.

Output Voltage

The regulated output voltage is calculated by :

Where VREF = 1.24V typical.

For most applications, R2 is a suggested a value up to

100k

Ω Place the resistor-divider as close to the IC as

possible to reduce the noise sensitivity.

Loop Compensation

The RT9277A/B voltage feedback loop can be compensated

with an external compensation network consisted of R3,

C3 and C4 (As shown in Figure 1). Choose R3 to set the

high-frequency integrator gain for fast transient response

without over or under compensation. Once R3 is

determined, C3 is selected to set the integrator zero to

maintain loop stability. The purpose of C4 is to cancel the

zero caused by output capacitor and the capacitor ESR. If

the ceramic capacitor is selected to be the output capacitor,

C4 can be taken off because of the small ESR. C2 is the

output capacitor as shown in Figure 1. The following

equations give approximate calculations of each

component :

1.2

IN(MIN)

OUT

OUT(MAX)

IN(MAX)

RIPPLE

IN(MAX)

PEAK

⎥

⎦

⎤

⎢

⎣

⎡

[]

OSC

OUT

OUT(MAX)

IN(MIN)

OUT

IN(MIN)

⎟

⎠

⎞

⎜

⎝

⎛

−

≅

OSC

OUT

PEAK

ESR

PEAK

RIPPLE_C

RIPPLE_ESR

RIPPLE

⎟

⎠

⎞

⎜

⎝

⎛ +

REF

OUT

RT9277APF Datasheet(PDF) 10 Page - Richtek Technology Corporation

RT9277APF Datasheet(HTML) 10 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12

Datasheet Download

Link URL