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RT8525

www.richtek.com

DS8525-01 March 2012

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Applications Information

The RT8525 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 RT8525 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. For typical application,

μF x 2, L1 = 33μH,

while the recommended value for compensation is as

Soft-Start

The soft-start of the RT8525 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

μAconstant 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