RT8805 데이터 시트보기 (PDF) - Richtek Technology
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RT8805 Datasheet PDF : 15 Pages
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Preliminary
RT8805
CLK1
PHASE1
PHASE2
MUX
RAMP1 RAMP2
CLK2
Current
Balance
OCP
-
S/H
GM
+
OC
IMAX
Figure 3. Current Sensing Loop
The sensing circuit gets IX = IL(S/H) x RDS(ON) x GM by
local feedback. IX is sampled and held just before low side
MOSFET turns off (See Figure 4). Therefore,
IX(S/H) = IL(S/H) x RDS(ON) x GM
IL(S/H)
=
IL(AVG)
−
VOUT
L
×
TOFF
2
,
TOFF
=
⎡
⎢⎣
VIN
− VOUT
VIN
⎤
⎥⎦
× 5μs,
FSW = 200kHz
IX (S/H)
=
⎡
⎢
⎢IL(AVG)
⎢
−
VOUT
× ⎜⎝⎛
VIN − VOUT
VIN
2L
⎟⎠⎞
×
5μs
⎤
⎥
⎥
⎥
⎢⎣
⎥⎦
×RDS(ON) × GM
Current Balance
RT8805 senses the voltage drop of the low-side MOS and
translates this to control the ramp signal. We can see
that the voltage signal finally injected to channel one is
proportional to (IL1 - IL2). Channel two is proportional to
(IL2 - IL1). In steady state and current balance situation,
there is no sensed signal injected into the ramp.
If IL1 > IL2, the ramp bottom of channel 1 will be lifted up
and decreased the duty of UGATE1. On the other hand,
the ramp bottom of channel 2 will be pulled low to increase
the duty of UGATE2. Finally, the loop will be back to the
balance state through above mentioned negative feedback
scheme. Figure 5 shows this scheme.
- COMP
VREF
+
+
RAMP2
-
Logic
&
Driver
VIN2
IL2
L2
VOUT
−1
2
RAMP1
k2 = k x RON2
VON2
VCSO2 = k2 x IL2 = k x VON2
VIN1
+
Logic
&
-
Driver
IL1
L1
CL
RL
VCSO1 = k1 x IL1
= k x VON1
k1 = k x RON1
VON1
Figure 5. Current Balance
Falling Slope = VOUT/L
IL
Inductor Current
IL(S/H)
IL(AVG)
High Side MOSFET Gate Signal
Low Side MOSFET Gate Signal
Figure 4. Inductor Current and Gate signals
Gate control
a. Before SS signal reach the valley of the ramp voltage,
UGATE and LGATE will be off.
b. If SS pin is pulled down 0.4V, UGATE and LGATE will
be off.
c. UV protect function caused by FB < 0.6V and SS >
3.7V, and controller will trigger Always Hiccup Mode.
d. When OC function occurs and SS > 3.7V, a constant
current of 10μA starts to discharge the capacitor
connected to SS pin right away. When OC occurs,
UGATE and LGATE will be off. 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 enable to pass to UGATE and
DS8805-01 November 2005
www.richtek.com
9
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