RT9605B 查看數據表（PDF） - Richtek Technology
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RT9605B Datasheet PDF : 13 Pages
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RT9605B
10
12V
1µF
PWM
1N4148
1µF
CBOOTx
BOOTX
VDD UGATEX
RT9605B
PHASEX
PWMX LGATEX
GND
12V
2N7002
CU
3nF
2N7002
20
CL
3nF
Figure 3. Test Circuit (One Phase is Shown)
Over Voltage Protection Function at Power On
An unique feature of the RT9605B is the addition of over
voltage protection in the event of upper MOSFET direct
shorted before power on. The RT9605B detects the fault
condition during initial start-up, the internal power on OVP
sense circuitry will rapidly drive the low side MOSFET on
before the multi-phase PWM controller takes control.
Figure 5 shows the measured waveforms with the high
side MOSFET directly shorted to 12V.
Figure 4 shows the power dissipation of the RT9605B as
a function of frequency and load capacitance. The value of
the CU and CL are the same and the frequency is varied
from 100kHz to 1MHz.
The operating junction temperature can be calculated from
the power dissipation curves (Figure 4). Assume
VDD = 12V, operating frequency is 200kHz and the
CU=CL=1nF which emulate the input capacitances of the
high side and low side power MOSFETs. From Figure 4,
the power dissipation is 100mW. For RT9605B, the
package thermal resistance θJA is 67°C/W, the operating
junction temperature is calculated as :
TJ = (67°C/W x 100mW) + 25°C = 31.7°C
(11)
where the ambient temperature is 25°C.
The method to improve the thermal transfer is to increase
the PC board copper area around the RT9605B firstly.
Then, adding a ground pad under IC to transfer the heat to
the peripheral of the board.
Power Dissipation vs. Frequency
1000
900
CU=CL=3nF
800
700
600
500
400
CU=CL=2nF
300
200
CU=CL=1nF
100
0
0
200
400
600
800
1000
Frequency (kHz)
Figure 4. Power Dissipation vs. Frequency
+12V
PHASEX
LGATEX
VCORE
Figure 5. Waveforms at High Side MOSFET Shorted
Please note that the +12V trigger point to RT9605B is at
3V, and the clamped level on PHASE pin is at about 2.4V.
Obviously since the PHASE pin voltage increases during
initial start-up, the VCORE increases correspondingly, but
it would quickly drop-off following the voltage in LGATE
and +12V.
Layout Consideration
Figure 6 shows the schematic circuit of a two-phase
synchronous buck converter to implement the either phase
of RT9605B. The converter operates at VIN 12V.
L1
1.2µH
VIN
12V
VCORE
C3
1500µF
12V
C1
1000µF
Q1
L2
2µH
Q2
R1
10
C4
1µF
D1
C2
1µF
CB BOOTX PVCCX
1µF
UGATEX PWMX
PHB83N03LT PHASEX
PHB95N03LT LGATEX GND
PWM
Figure 6. Sync. Buck Converter Circuit
DS9605B-03 March 2011
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www.richtek.com
11
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