Rainbow Electronics MAX5098A - Manual (Page 21)

MAX5098A
Dual, 2.2MHz, Automotive Buck or Boost
Converter with 80V Load-Dump Protection
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the capacitor discharge, and ΔV ESR is the contribution
due to the ESR of the capacitor. D MAX is the maximum
duty cycle at minimum input voltage.
Power Dissipation
The MAX5098A includes two internal power MOSFET
switches. The DC loss is a function of the RMS current in
the switch while the switching loss is a function of switch-
ing frequency and instantaneous switch voltage and cur-
rent. Use the following equations to calculate the RMS
current, DC loss, and switching loss of each converter.
The MAX5098A is available in a thermally enhanced
package and can dissipate up to 2.7W at +70°C ambient
temperature. The total power dissipation in the package
must be limited so that the operating junction tempera-
ture does not exceed its absolute maximum rating of
+150°C at maximum ambient temperature.
For the buck converter:
where
See the Electrical Characteristics table for the
R ON(MAX) maximum value.
For the boost converter:
where V DS is the drop across the internal MOSFET and
η is the efficiency. See the Electrical Characteristics
table for the R ON(MAX) value.
where t
R
and t
F
are rise and fall times of the internal
MOSFET. t F can be measured in the actual application.
The supply current in the MAX5098A is dependent on
the switching frequency. See the
Typical Operating
Characteristics to find the supply current of the
MAX5098A at a given operating frequency. The power
dissipation (P
S ) in the device due to supply current
(I SUPPLY ) is calculated using following equation.
P S = V INMAX x I SUPPLY
The total power dissipation P T in the device is:
P T = P DC1 + P DC2 + P SW1 + P SW2 + P S
where P DC1 and P DC2 are DC losses in converter 1 and
converter 2, respectively. P
SW1
and P
SW2
are switching
losses in converter 1 and converter 2, respectively.
Calculate the temperature rise of the die using the fol-
lowing equation:
T J = T C x (P T x θ
JC )
where θ
JC
is the junction-to-case thermal impedance of
the package equal to +1.7°C/W. Solder the exposed
pad of the package to a large copper area to minimize
the case-to-ambient thermal impedance. Measure the
temperature of the copper area near the device at a
worst-case condition of power dissipation and use
+1.7°C/W as
θ
JC
thermal impedance.
Compensation
The MAX5098A provides an internal transconductance
amplifier with its inverting input and its output available
for external frequency compensation. The flexibility of
external compensation for each converter offers wide
selection of output filtering components, especially the
output capacitor. For cost-sensitive applications, use
aluminum electrolytic capacitors; for component size-
sensitive applications, use low-ESR tantalum, polymer,
or ceramic capacitors at the output. The high switching
frequency of MAX5098A allows use of ceramic capaci-
tors at the output.
Choose all the passive power components that meet
the output ripple, component size, and component cost
requirements. Choose the small-signal components for
the error amplifier to achieve the desired closed-loop
P
V I t t f
SW
O IN R F SW =
Ч Ч +
( )
Ч
4
I I I I I
D
I
V I
V
I
V V D
L f
I I
I
I I
I
P I R
RMS DC PK DC PK
MAX
IN
O O
IN
L
IN DS
SW
DC IN
L
PK IN
L
DC RMS DS ON MAX
= + + Ч
( )
( )
Ч
=
Ч
Ч
=
( )
Ч
Ч
=
= +
= Ч
−
−
2 2
2
3
2
2
η
Δ
Δ
Δ
( )( )
I I
I
I I
I
P
V I t t f
DC O
L
PK O
L
SW
IN O R F SW
=
= +
=
Ч Ч +
( )
Ч
−
Δ
Δ
2
2
4
I I I I I
D
P I R
RMS DC PK DC PK
MAX
DC RMS DS ON MAX
= + + Ч
( ) ⎛
⎝
⎞
⎠ Ч
= Ч
2 2
2
3
( )