7

RT8525

DS8525-01 March 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

duty.

Output Over Voltage Protection

The output voltage can be clamped at the voltage level

determined by the following equation :

⎛⎞

×⎜⎟

⎝⎠

OVP1

OUT (OOVP)

OOVP

OVP2

OOVP

R

V

= V

1+

,

R

where V

= 2.5V (typ.)

Power MOSFET Selection

For the applications operating at high output voltage,

switching losses dominate the overall power loss.

Therefore, the power N-MOSFET switch is typically

chosen for drain voltage, VDS, rating and low gate charge.

secondary. The VDC regulator in the RT8525 has a fixed

output current limit to protect the IC and provide 12V DRV

voltage for N-MOSFET switch gate driver.

()

×−

×

××

2

OUT

SW

OUT

D1 D

V

L =

2f

I

Inductor Selection

The boundary value of the inductance L between

Discontinuous Conduction Mode (DCM) and Continuous

Conduction Mode (CCM) can be approximated by the

following equation :

Figure 2. Fault Protection Function Block

where

fsw is the operating frequency,

and D is the duty cycle calculated by the following

equation :

−

OUT

IN

OUT

VV

D =

V

The boost converter operates in DCM over the entire input

voltage range if the inductor value is less than the boundary

value L. With an inductance greater than L, the converter

operates in CCM at the minimum input voltage and may

transit to DCM at higher voltages. The inductor must be

Action 1

Action 2

Node A

Comparator 1

Comparator 2

1.25V

+

-

+

-

+

0.25V

+

8k

OOVP

OUVP, OTP

Switch 1

Switch 2

12V

100k

FAULT

−×

×

MAX

SLP

S

L, PK

0.4 D

R

50μ

R <

I

be under 0.25V. Then the protection function will perform

action 2 to turn off the driver. When protection function is

released, the RT8525 will re-start.

On the other hand, if the triggered protection is OOVP,

the voltage at node A will be decided by voltage divider

Ω resistor. This

voltage must be designed between 0.25V and 1.25V by

Switch 2, the divided FAULT voltage will activate action 1

to turn off the driver without resetting soft-start. Therefore,

when protection function OOVP is released, the RT8525

will be in normal operation.

to the OOVP pin.

Fault Protection

The FAULT pin will be pulled low once a protection is

Ω if the pulled-high voltage was

12V. The following figure illustrates the fault protection

function block. If one of the OUVP and OTP occurs, the

switch 1 will be turned on, and the voltage at node A will