MB39C011APFT-G-BND-ERE1 Datasheet, PDF (22/53 Pages) Cypress Semiconductor – 2 ch DC/DC Converter IC with Synchronous Rectification

MB39C011A
11. Setting The Triangular Oscillation Frequency
The triangular oscillation frequency is determined by the timing resistor (R_T) connected to the RT pin (pin 5).
fosc 
0.001
122.4 10 ^{ 12} R_T 10³  96 10 ^{ 9}
fosc : Triangular oscillation frequency [kHz]
R_T : Timing resistance [k]
The upper limit on the oscillation frequency that can be set depends on the junction temperature and duty cycle. It is recommended
that the device is used within the range shown in the following graph.
Oscillation frequency vs. Junction temperature
2100
1900
1700
1500
1300
1100
900
700
500
300
100
-30
0
+30 +60 +90 +120
Junction temperature Tj (˚C)
Note : Refer to “ Power Dissipation and Thermal Design” for details on calculating the junction temperature.
11.1 Power Dissipation and Thermal Design
It is necessary to examine it for the use at a high power-supply voltage, a high oscillation frequency, and the high temperature.
Also use within the range of “Oscillation frequency vs. Junction temperature”.
The junction temperature can be investigated from the internal power dissipation of the IC.
The internal power dissipation of the IC (P_IC) is given by the following formula.
P_IC  V_CC  (I_CC  Qg fosc)
P_IC : Internal IC power dissipation [W]
V_CC : Power supply voltage (VIN : [V])
I_CC : Power supply current (3.3 mA Max)
Qg : Total electric charge (Vgs  5 V) of all SW FET for 2ch [C]
fosc : Oscillation frequency [Hz]
The junction temperature is given by the following formula.
Document Number: 002-08369 Rev. *A
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