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PDF RT8805C Data sheet ( Hoja de datos )
| Número de pieza | RT8805C | |
| Descripción | Two Phase General Purpose PWM Controller | |
| Fabricantes | Richtek Technology | |
| Logotipo |  |
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Preliminary
RT8805C
Two Phase General Purpose PWM Controller
General Description
The RT8805C is the most compact dual-phase
synchronous buck controller in the industry specifically
designed for high power density applications. This part is
capable of delivering up to 60A output current due to its
embedded bootstrapped drivers that support 12V + 12V
www.DataSdherievti4nUg.ccoampability.
The phase currents are sensed by innovative time sharing
RDS(ON) current sensing technique for current balance and
over current balance. Using one common GM amplifier to
sense two phase currents eliminates offset and
nonlinearity of the GM amplifier and yields good current
balance. Other features include adjustable operation
frequency from 50kHz to 1MHz, adjustable soft-start,
PGOOD, external compensation, enable/shutdown for
various application and performance consideration.
The RT8805C comes to a tiny footprint package of
VQFN-16L 3x3 and VQFN-24L 4x4 packages.
Ordering Information
RT8805C
Package Type
QV : VQFN-16L 3x3 (V-Type)
QVA : VQFN-24L 4x4 (V-Type)
Operating Temperature Range
P : Pb Free with Commercial Standard
G : Green (Halogen Free with Commer-
cial Standard)
Note :
Richtek Pb-free and Green products are :
`RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
`Suitable for use in SnPb or Pb-free soldering processes.
`100% matte tin (Sn) plating.
Features
z 12V Power Supply Voltage
z 2 Phase Power Conversion
z Embedded 12V Boot Strapped Driver
z Precise Core Voltage Regulation
z Low Side MOSFET RDS(ON) Current Sensing for
Power Stage Current Balance
z External Compensation
z Adjustable Soft-Start
z Adjustable Frequency and Typical at 300kHz Per
Phase
z Power Good Indication
z Adjustable Over Current Protection
z External Reference Voltage Tracking (RT8805CxQVA)
z Small 16-Lead and 24-Lead VQFN Packages
z RoHS Compliant and 100% Lead (Pb)-Free
Applications
z Middle-High End GPU Core Power
z High End Desktop PC Memory Core Power
z Low Output Voltage, High Power Density DC-DC
Converters
z Voltage Regulator Modules
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area, otherwise visit our website for detail.
DS8805C-03 August 2007
www.richtek.com
1
1 page  
Preliminary
RT8805C
Functional Pin Description
Pin No.
VQFN-16L 3x3 VQFN-24L 4x4
16 1
Pin Name
PHASE1
12 19 PHASE2
1 2 UGATE1
11 18 UGATE2
2
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10
3 BOOT1
17 BOOT2
3 4 AGND
4 5 IMAX
-- 6, 8, 9, 10, 24 NC
5 7 RT
Exposed Pad (17)
11,
Exposed Pad (25)
GND
6 12 SS
7 13 COMP
8 14 FB
9 16 PGOOD
15 23 LGATE1
13 20 LGATE2
14
21, 22
VCC
-- 15 PI
Pin Function
These pins are return nodes of the high-side driver. Connect
These pins to high-side MOSFET sources together with the
low-side MOSFET drains and the inductors.
Upper Gate Drive. These pins drive the gates of the high
side MOSFETs.
Bootstrap Power Pin. These pins power the high-side
MOSFET drivers. Connect These pins to the junctions of the
bootstrap capacitors.
Chip Analog Ground.
Maximum Current Setting. This pin sets the current limiting
level. Connect this pin with resistor to ground to set the
current limit.
No Internal Connection.
Timing Resistor. Connect a resistor from RT to AGND to set
the clock frequency.
The exposed pad must be soldered to a large PCB and
connected to GND for maximum power dissipation.
Soft-Start Pin. This pin provides soft-start function for
controller. The COMP voltage of the converter follows the
ramping voltage on the SS pin.
Compensation Pin. This pin is output node of the error
amplifier.
Feedback Pin. This pin is negative input pin of the error
amplifier.
Power Good. PGOOD is an open drain output used to
indicate the status of the voltages on SS pin and FB pin.
PGOOD will go high impedance when SS > 3.7V and FB >
0.6V.
Lower Gate Drive. These pins drive the gate of the lowside
MOSFETs.
The VCC pin is the external 12V power. Internal 5V power
(VDD) is regulated from this pin. This pin also powers the low
side MOSFET drivers.
External reference voltage pin. This pin sets the voltage of
FB pin when close loop.
DS8805C-03 August 2007
www.richtek.com
5
5 Page 
Preliminary
RT8805C
CLK1
PHASE1
PHASE2
MUX
RAMP1 RAMP2
CLK2
Current
Balance
OCP
S/H
OC
-
GM
+
www.DataSheet4U.com
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
RT8805C 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.
VREF
-
+
RAMP2
COMP
+
-
Logic
&
Driver
VIN2
IL2
L2
−
1
2
RAMP1
k2 = k x RON2
VON2
VCSO2 = k2 x IL2 = k x VON2
VIN1
Logic
+&
- Driver
IL1
L1
VOUT
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
DS8805C-03 August 2007
www.richtek.com
11
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet RT8805C.PDF ] | |
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