9

RT8525D

DS8525D-00 June 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 (OVP)

OVP

OVP2

OVP

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 RT8525D 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

selected with a saturated current rating greater than the

peak current provided by the following equation :

×

××

+

××

η

OUT

OUT

LPK

IN

VI

VIN D T

I

=

V2 L

where

η is the efficiency of the power converter.

−×

×

MAX

SLP

S

L, PK

0.4 D

R

50μ

R <

I

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

released, the RT8525D will re-start.

On the other hand, if the triggered protection is OVP, 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 OVP is released, the RT8525D

will be in normal operation.

to the OVP 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

be under 0.25V. Then the protection function will perform

Action 1

Action 2

Node A

Comparator 1

Comparator 2

1.25V

+

-

+

-

+

0.25V

+

8k

OVP

OUVP, OTP

Switch 1

Switch 2

12V

100k

FAULT