RT9624B Datasheet, PDF (12/16 Pages) Richtek Technology Corporation – Single Phase Synchronous Rectified Buck MOSFET Driver

RT9624B
BOOT
UGATE
PHASE
V_CC
V_IN
C_BOOT
+
V_CB
-
LGATE
GND
Figure 2. Part of Bootstrap Circuit of RT9624B
In practice, a low value capacitor C_BOOT will lead to the
over charging that could damage the IC. Therefore, to
minimize the risk of overcharging and to reduce the ripple
on V_CB, 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.
It is recommended to adopt a ceramic or tantalum
capacitor.
Power Dissipation
To prevent driving the IC beyond the maximum
recommended operating junction temperature of 125°C,
it is necessary to calculate the power dissipation
appropriately. This dissipation is a function of switching
frequency and total gate charge of the selected MOSFET.
Figure 3 shows the power dissipation test circuit. C_L and
C_U are the UGATE and LGATE load capacitors,
respectively. The bootstrap capacitor value is 1μF.
C_BOOT
1µF
12V
10
12V
1µF
PWM
BOOT
VCC UGATE
RT9624B
PHASE
PWN
LGATE
GND
2N7002
C_U
3nF
2N7002
20
C_L
3nF
Figure 3. Power Dissipation Test Circuit
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
Figure 4 shows the power dissipation of the RT9624B as
a function of frequency and load capacitance when V_CC =
12V. The value of C_U and C_L are the same and the frequency
is varied from 100kHz to 1MHz.
Power Dissipation vs. Frequency
1000
900
800
CU = CL = 3nF
700
600
CU = CL = 2nF
500
400
300
200
CU = CL = 1nF
100
0
0
VCC = 12V
200
400
600
800
1000
Frequency (kHz)
Figure 4. Power Dissipation vs. Frequency
The operating junction temperature can be calculated from
the power dissipation curves (Figure 4). Assume
V_CC = 12V, operating frequency is 200kHz and C_U = C_L =
1nF which emulate the input capacitances of the high side
and low side power MOSFETs. From Figure 4, the power
dissipation is 100mW. Thus, for example, with the SOP-
8 package, the package thermal resistance θ_JA is 120°C/
W. The operating junction temperature is then calculated
as :
T_J = (120°C/W x 100mW) + 25°C = 37°C
(11)
where the ambient temperature is 25°C.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction to ambient
thermal resistance.
is a registered trademark of Richtek Technology Corporation.
DS9624B-04 October 2012