8

RT8525D

www.richtek.com

DS8525D-00 June 2012

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Applications Information

The RT8525D is a wide input operating voltage range step

up controller. High voltage output and large output current

are feasible by using an external N-MOSFET. The

protection functions include output over voltage, output

under voltage, over temperature and current limiting

protection.

Boost Output Voltage Setting

The regulated output voltage is set by an external resistor

divider according to the following equation :

⎛⎞

×⎜⎟

⎝⎠

FB1

OUT

FB

FB

FB2

R

V

= V

1+

, where V

= 1.25V (typ.)

R

Ω

for saving sacrificing. Moreover, placing the resistor divider

as close as possible to the chip can reduce noise

sensitivity.

Boost Switching Frequency

The RT8525D boost driver switching frequency is able to

Ω to

220k

Ω. The following figure illustrates the corresponding

switching frequency within the resistor range.

Figure 1. Boost Switching Frequency

0

100

200

300

400

500

600

0

50

100

150

200

250

Ω

Boost Loop Compensation

The voltage feedback loop can be compensated by an

and pole to maintain loop stability. A typical compensation

for the RT8525D is choosing 3k

Soft-Start

The soft-start of the RT8525D can be achieved by

connecting a capacitor from the SS pin to GND. The built-

in soft-start circuit reduces the start-up current spike and

output voltage overshoot. The external capacitor charged

by an internal 4

μA constant charging current determines

the soft-start time. The SS pin limits the rising rate of the

COMP pin voltage and thereby limits the peak switch

current. The soft-start interval is set by the soft-start

capacitor according to the following equation :

SS

SS

tC

5 10

A typical value for the soft-start capacitor is 0.33

μF. The

soft-start capacitor is discharged when EN voltage falls

below its threshold after shutdown delay or UVLO occurs.

Slope Compensation and Current Limiting

A slope compensation is applied to avoid sub-harmonic

oscillation in current-mode control. The slope

compensation voltage is generated by the internal ramp

The inductor current is sensed by the sensing resistor

pin. The internal ramp current is rising linearly form zero

at the beginning of each switching cycle to 50

μA in

maximum on-time of each cycle. The slope compensation

The current flow through inductor during charging period

compensation voltage also attributes magnitude to ISW.

As the voltage at the ISW pin is over 0.4V, the DRV will

be pulled low and turn off the external N-MOSFET. So

that the inductor will be forced to leave charging stage

and enter discharging stage to prevent over current. The

current limiting can be calculated by the following equation:

()

−×

××

×

OUT

IN

S

SLP

SW

VV

R

R

>

2L 50μ f