RT9605A データシートの表示（PDF） - Richtek Technology
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RT9605A Datasheet PDF : 12 Pages
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Preliminary
RT9605A
In practice, a low value capacitor CB will lead the over-
charging that could damage the IC. Therefore to minimize
the risk of overcharging and reducing the ripple on VCB,
the bootstrap capacitor should not be smaller than 0.1μF,
and the larger the better. In general design, using 1μF can
provide better performance. At least one low-ESR capacitor
should be used to provide good local de-coupling. Here,
to adopt either a ceramic or tantalum capacitor is suitable.
Power Dissipation
For not exceeding the maximum allowable power
dissipation to drive the IC beyond the maximum
recommended operating junction temperature of 125°C,
it is necessary to calculate power dissipation appro-
priately. This dissipation is a function of switching
frequency and total gate charge of the selected MOSFET.
Figure 4 shows the power dissipation test circuit. CL and
CU are the UGATE and LGATE load capacitors,
respectively. The bootstrap capacitor value is 1μF.
+12V
10
1uF
PWM
1N4148
1uF
CBOOTx
BOOTX
VDD UGATEX
RT9605A
PHASEX
PWMX LGATEX
GND
+12V
2N7002
CU
3nF
2N7002
20
CL
3nF
The method to improve the thermal transfer is to increase
the PC board copper area around the RT9605A 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 5. Power Dissipation vs. Frequency
Over-Voltage Protection Function at Power-On
An unique feature of the RT9605A is the addition of over-
voltage protection in the event of upper MOSFET direct
shorted before power-on. The RT9605A 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 6 shows the measured waveforms with the high
side MOSFET directly shorted to 12V.
Figure 4. Test Circuit (One Phase is Shown)
Figure 5 shows the power dissipation of the RT9605A 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.
+12V
PHASEX
The operating junction temperature can be calculated from
the power dissipation curves (Figure 5). Assume
VDD = 12V, operating frequency is 200kHz and the
LGATEX
VCORE
CU=CL=1nF which emulate the input capacitances of the
high side and low side power MOSFETs. From Figure 5,
Figure 6. Waveforms at High Side MOSFET Shorted
the power dissipation is 100mW. For RT9605A, the Please note that the +12V trigger point to RT9605A is at
package thermal resistance θJA is 67°C/W, the operating
junction temperature is calculated as :
3V, and the clamped level on PHASE pin is at about 2.4V.
Obviously since the PHASE pin voltage increases during
TJ = (67°C/W x 100mW) + 25°C = 31.7°C
where the ambient temperature is 25°C.
(11)
initial start-up, the VCORE increases correspondingly, but
it would quickly drop-off following the voltage in LGATE
and +12V.
All brandname or trademark belong to their owner respectively
DS9605A-05 August 2007
www.richtek.com
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