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RT8805

Two Phase General Purpose PWM Controller

Richtek Technology 

RT8805

Preliminary

When the voltage at the capacitor connected to SS pin

pass about 0.4V, a constant current of 10μA starts to

charge the capacitor. The PWM signal is enabled to pass

to the UGATE and LGATE. OCP function monitors both

channels, either one can activate OCP. If the OC protection

occurs three times, the chip will shut down and the state

will only be released by POR.

RT8805 uses an external resistor RIMAX to set a

programmable over current trip point. OCP comparator

compares each inductor current with this reference current.

RT8805 uses hiccup mode to eliminate fault detection of

OCP or reduce output current when output is shorted to

ground. The OCP comparator compares the difference

between IX and IIMAX.

OCP Comparator

IIMAX

+

IX

-

For example:

From Electrical Specifications : RIMAX = 33kΩ

VPHASE = -220mV

Assume Low side MOSFET RDS(ON) = 3mΩ.

Get the OCP setting current is 232m0mΩV=73A per PHASE

(the valley of inductor's current).

Change the setting current which you want from 73A per

PHASE to 50A per PHASE.

Following below steps:

1. Calculate phase voltage. If Low side MOSFET

RDS(ON) = 3mΩ, VPHASE_new = -150mV.

2. RIMAX_new

=

-220mV

VPHASE_new

× 33kΩ

RIMAX_new = 48.4kΩ

UVP

By detecting voltage at FB pin when SS > 3.7V. If

FB < 0.6V, the chip will trigger the always Hiccup mode

and a constant current source 10μA starts to charge

capacitor at SS pin when SS pass 0.4V and discharge

Css when SS > 3.7V. As Figure 12 shown.

VOUT

(20mV/Div)

VIN = 0V

UVP

SS

(2V/Div)

LGATE

(10V/Div)

UGATE

(1V/Div)

Time (50ms/Div)

Figure 12. UVP (Always Hiccup Mode)

OTP

Monitor the temperature near the driver part within the

chip. Shutdown the chip when OTP (Typical trip point :

170°C).

General Design Guide

This design guide is intended to provide a high-level

explanation of the steps necessary to create a multi-phase

power converter. It is assumed that the reader is familiar

with many of the basic skills and techniques referenced

below.

Power Stages

Designing a multi-phase converter is to determine the

number of phases. This determination depends heavily

on the cost analysis which in turn depends on system

constraints that differ from one design to the next.

Principally, the designer will be concerned with whether

components can be mounted on both sides of the circuit

board, whether through-hole components are permitted,

the total board space available for power-supply circuitry,

and the maximum amount of load current. Generally

speaking, the most economical solutions are those in

which each phase handles between 20 to 25 A (One Upper

and one Lower MOSFET). All surface-mount designs will

tend toward the lower end of this current range.

If through-hole MOSFETs and inductors can be used,

higher per-phase currents are possible. In cases where

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DS8805-01 November 2005

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