FA551x ETC, FA551x Datasheet - Page 20

• Manufacturer Part Number: FA551x
• Description: CMOS IC For Switching Power Supply Control
• Manufacturer: ETC

FA551X
15. Loss calculation
IC loss must be confirmed to use the IC within the ratings.
Since it is hard to directly measure IC loss, some examples of
calculating approximate IC loss are given below.
15.1 Calculation example 1
Suppose the supply voltage is Vcc, IC current consumption is
lccop, the total gate charge of the power MOSFET is Qg, and
the switching frequency is f
calculated by:
Pd
This expression calculates an approximate value of Pd, which is
normally a little larger than the actual loss. Since various
conditions such as temperature characteristics apply,
thoroughly verify the appropriateness of the calculation under
all applicable conditions.
Example:
When Vcc=18V, lccop=2.5mA (max.) is obtained from the
specifications. Suppose Qg=80nC and fsw=100kHz.
Pd
15.2 Calculation example 2
The IC loss consists of the loss caused by operation of the
control circuit and the loss caused at the output circuit to drive
the power MOSFET.
15.2.1 Loss at the control circuit
The loss caused by operation of the IC control circuit is
calculated by the supply voltage and IC current consumption.
When the supply voltage is Vcc and IC current consumption is
lccop, loss Pop at the control circuit is:
Pop = V
Example:
When Vcc=18, lccop=1.9mA (typ) is obtained from the
specifications. The typical IC loss is given by:
Pop = 18V
15.2.2 Loss at the output circuit
The output circuit of the IC is a MOSFET push-pull circuit.
When the ON resistances of MOSFETs making up the output
circuit are Ron and Roff, the resistances can be determined as
shown below based on Vcc=18V and Tj = 25˚C obtained from
the output characteristics shown in the specifications:
Ron = 15 (typ), Roff=7 (typ)
When the total gate charge of the power MOSFET is Qg, the
switching frequency is f
resistance is Rg, the loss caused at the IC output circuit is given
by:
Pdr =
20
V
18V
189mW
2
1
CC
CC
V
CC
(I
(2.5mA + 80nC
I
1.5mA = 27mW
CCOP
CCOP
Qg fsw
+ Qg
........................................................... (13)
SW
, the supply voltage is Vcc, and gate
SW
fsw)
(
. Total IC loss Pd can be
Rg + Ron
100kHz)
........................................ (12)
Ron
+
Rg + Roff
Roff
)
...... (14)
When gate resistance differs between ON and OFF as shown
in Figure 36, the loss is given by:
Pdr =
Example:
When Vcc=18V, Qg=80nC, fsw=100kHz, and Rg=10 , the
typical IC loss is given by:
Pdr =
15.2.3 Total loss
The total loss (Pd) of the IC is the sum of the control circuit loss
(Pop) and the output circuit loss (Pdr) calculated previously:
Pd = Pop + Pdr
Example:
The standard IC loss under the conditions used in (1) and (2)
above are:
Pd = Pop + Pdr = 27mW + 72.8mW = 99.8mW
=72.8mW
2
2
1
1
V
18V
CC
.................................................................. (16)
Qg fsw
80nC
Fig. 35
Fig. 36
6
4
Qon
Qoff
6
4
VCC
GND
Qon
Qoff
VCC
GND
(
100kHz
Rg1+Rg2+Ron Rg1+Roff
5
Output Circuit (1)
Output Circuit (2)
OUT
5
OUT
Rg1
Ron
Rg
Rg2
(
10 +15
15
+
Roff
+
10 +7
)
7
.... (15)
)