MB39A123 Datasheet, PDF (28/44 Pages) Fujitsu Component Limited. – 6 ch DC/DC Converter IC with Synchronous Rectification

MB39A123
■ SETTING THE DEAD TIME (ch.2 to ch.6)
When the device is set for step-up or inverted output based on the step-up, step-up/down Zeta method, step up/
down Sepic method, or flyback method, the FB terminal voltage may reach and exceed the triangular wave
voltage due to load fluctuation. If this is the case, the output transistor is fixed to a full-ON state (ON duty =
100%). To prevent this, set the maximum duty of the output transistor.
When the DTC terminal is opened, the maximum duty is 92% (Typ) because of this IC built-in resistor which
sets the DTC terminal voltage. This is based on the following setting: 1MHz (RT = 6.8kΩ/CT = 100pF).
To disable the DTC terminal, connect it to the VREF terminal (pin 9) as illustrated below (when dead time is not
set).
• When dead time is set:
(Setting with built-in resistor:
1MHz [RT = 6.8kΩ/CT = 100pF] =: 92%)
• When dead time is not set:
9 VREF
“Open”
DTCX
DTCX
X : ch.2 to ch.6
X : ch.2 to ch.6
To change the maximum duty using external resistors, set the DTC terminal voltage by dividing resistance using
the VREF voltage. Refer to “• When dead time is set : (Setting by external resistors)”.
It is possible to set without regard for the built-in resistance value (including tolerance) when setting the external
resistance value to 1/10 of the built-in resistance or less.
Note that the VREF load current must be set such that the total current for all the channels does not exceed 1 mA.
When the DTC terminal voltage is higher than the triangular wave voltage, the output transistor is turned on.
The formula for calculating the maximum duty is as follows, assuming that the triangular wave amplitude and
triangular wave lower limit voltage are about 0.5 V and 0.4 V, respectively.
Vdt − 0.4 V
DUTY (ON) Max =:
× 100 (%)
0.5 V
Rb
R1
R2
Vdt =
× VREF (V) (condition : Ra <
, Rb <
)
Ra + Rb
10
10
Note : DUTY obtained by the above-mentioned formula is a calculated value. For setting, refer to “ON Duty cycle
vs. DTC terminal voltage”.
The maximum duty varies depending on the oscillation frequency, regardless of settings in built-in or external
resistors.
(This is due to the dependency of the peak value of a triangular wave on the oscillation frequency and RT.
Therefore, if RT is greater, the maximum duty decreases, even when the same frequency is used.)
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