Rainbow Electronics MAX5081 - Manual (Page 15)

MAX5080/MAX5081
1A, 40V, MAXPower Step-Down
DC-DC Converters
______________________________________________________________________________________ 15
Since R3 >> R6, R3 + R6 can be approximated as R3.
R3 is then calculated as:
f P3 is set at 5xf C . Therefore, C8 is calculated as:
Power Dissipation
The MAX5080/MAX5081 is available in a thermally
enhanced package and can dissipate up to 2.7W at T
A
= +70°C. When the die temperature reaches +160°C,
the part shuts down and is allowed to cool. After the
parts cool by 20°C, the device restarts with a soft-start.
The power dissipated in the device is the sum of the
power dissipated from supply current (P
Q ), transition
losses due to switching the internal power MOSFET
(P
SW ), and the power dissipated due to the RMS cur-
rent through the internal power MOSFET (P
MOSFET
).
The total power dissipated in the package must be lim-
ited such that the junction temperature does not
exceed its absolute maximum rating of +150°C at maxi-
mum ambient temperature. Calculate the power lost in
the MAX5080/MAX5081 using the following equations:
The power loss through the switch:
P
MOSFET
= I
RMS_MOSFET 2 x R
ON
R ON is the on-resistance of the internal power MOSFET
(see Electrical Characteristics).
The power loss due to switching the internal MOSFET:
where t
R
and t
F
are the rise and fall times of the internal
power MOSFET measured at LX.
The power loss due to the switching supply current
(I
SW
):
P Q = V IN x I SW
The total power dissipated in the device will be:
P
TOTAL
= P
MOSFET
+ P
SW
+ P
Q
Chip Information
TRANSISTOR COUNT: 4300
PROCESS: BiCMOS/DMOS
( )
P
f
SW
SW
=
Ч Ч Ч Ч
V I t t
IN OUT R F
4
( )
_
P I x R
I I I I I
D
I I
I
I I
I
MOSFET RMS MOSFET ON
RMS MOSFET PK PK DC DC
PK OUT
P P
DC OUT
P P
=
= + Ч +
⎡
⎣⎢
⎤
⎦⎥
Ч
= +
= −
−
−
2
2 2
3
2
2
_
∆
∆
( )
C
C
C R f
8
7
2 7 5 1
=
Ч Ч Ч − π
P3
R
f C
3
1
2 6
≈
Ч Ч π
LC
GAIN
(dB)
V OUT
REF
R3
COMP
R6
R5
C6
R4
FREQUENCY
CLOSED-LOOP
GAIN
EA
GAIN
f Z1 f Z2 f C f P2 f P3
C8
EA
C7
Figure 4. Error Amplifier Compensation Circuit (Closed-Loop
and Error Amplifier Gain Plot) for Higher ESR Output Capacitors