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參數(shù)資料
| 型號(hào)： | RT8805 |
| 廠商： | Richtek Technology Corporation |
| 英文描述： | Two Phase General Purpose PWM Controller |
| 中文描述： | 兩相通用PWM控制器 |
| 文件頁數(shù)： | 12/15頁 |
| 文件大?。?/td> | 387K |
| 代理商： | RT8805 |
RT8805
Preliminary
12
DS8805-01 November 2005
www.richtek.com
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
+
-
I
IMAX
I
X
OCP Comparator
For example:
From
Electrical Specifications
: R
IMAX
= 33k
Ω
V
PHASE
= -220mV
Assume Low side MOSFET R
DS(ON)
= 3m
Ω
.
Get the OCP setting current is =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
R
DS(ON)
= 3m
Ω
, V
PHASE_new
= -150mV.
UVP
By detecting voltage at FB pin when SS > 3.7
V
. If
FB < 0.6
V
, the chip will trigger the always Hiccup mode
and a constant current source 10
μ
A starts to charge
capacitor at SS pin when SS pass 0.4V and discharge
Css when SS > 3.7V. As Figure 12 shown.
Figure 12. UVP (Always Hiccup Mode)
UVP
Time (50ms/Div)
V
OUT
(20mV/Div)
LGATE
(10V/Div)
SS
(2V/Div)
UGATE
(1V/Div)
V
IN
= 0V
When the voltage at the capacitor connected to SS pin
pass about 0.4V, a constant current of 10
μ
A 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 R
IMAX
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 I
X
and I
IMAX
.
3m
Ω
=
Ω
×
=
48.4k
R
33k
V
-220mV
PHASE_new
R
2.
IMAX_new
IMAX_new
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