FA3686 Datasheet, PDF (17/18 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

3. Determining the output voltage of DC-DC converters
The ways to determine the output voltage of the DC-DC
converter of each channel is shown in Fig. 10 and the following
equations.
For ch1:
The output voltage of a boost circuit is determined by:
Vout1 = ^{R1 + R2} Ï« VREF
R2
For ch2:
The output voltage of an inverting circuit is determined by:
Vout2 =
R3 + R4
R3
ϫ VREF –
R4
R3
Ï« VREG
The ratio of resistances is determined by:
R3 ₌ VREG – VREF
R3 Vout2 + VREF
(Use the absolute value of the Vout2 voltage.)
For ch3:
The output voltage of a series regulator is determined by:
Vout3 = ^{R5 + R6} Ï« VREF
R6
4. Restriction of external discrete components and
recommended operating conditions
To achieve a stable operation of the IC, the value of external
discrete components connected to VCC, VREG, CS pins should
be within the recommended operating conditions. And the
voltage and current applied to each pin should be also within the
recommended operating conditions. If the pin voltage of OUT1,
OUT2, or VREG becomes higher than the VCC pin voltage, the
current flows from the pins to the VCC pin because parasitic
three diode exist between the VCC pin and these pins. Be careful
not to allow this current to flow.
5. Loss calculation of IC
Since it is difficult to measure IC loss directly, the calculation to
obtain the approximate loss of the IC connected directly to a
MOSFET is described below.
When the supply voltage is VCC, the current consumption of the
IC is ICCA, the total input gate charge of the driven MOSFET is
Qg and the switching frequency is fsw, the total loss Pd of the IC
can be calculated by:
Pd Ô¼ VCC Ï« (ICCA + Qg Ï« fsw).
The value in this expression is influenced by the effects of the
dependency of supply voltage, the characteristics of temperature,
or the tolerance of parameter. Therefore, evaluate the
appropriateness of IC loss sufficiently considering the range of
values of above parameters under all conditions.
Example:
ICCA=3.0mA for VCC=3.3V in the case of a typical IC from the
characteristics curve. Qg=6nC, fsw=500kHz, the IC loss “Pd” is
as follows.
Pd Ô¼ 3.3 Ï« (3.0mA + 6nC Ï« 500kHz) Ô¼ 19.8mW
If two MOSFETs are driven under the same condition for 2
channels, Pd is as follows:
Pd Ô¼ 3.3 Ï« {3.0mA + 2 Ï« (6nC Ï« 500kHz)} = 29.7mW
Vout1
OUT1
9
R1
IN1-
14
+
R2
VREF
(1.0V)
15 FB1
VREG
13
R3
IN2-
4
R4
Vout2
OUT2
8
+
VREF
(1.0V)
3 FB2
Vout3
R5
IN3-
2
+
R6
VREF
(1.0V)
FB3
1
Fig. 10
FA3686V
Vout1
Vout2
Vout3
17