Rainbow Electronics MAX5099 User Manual
Page 20
MAX5099
Dual, 2.2MHz, Automotive Synchronous Buck
Converter with 80V Load-Dump Protection
20
______________________________________________________________________________________
where
where V
DS
is the voltage drop across the internal MOSFET
switch. ΔI
L
is the peak-to-peak inductor ripple current
as calculated above. ΔV
Q
is the portion of input ripple
due to the capacitor discharge, and ΔV
ESR
is the con-
tribution due to ESR of the capacitor.
Output Capacitor
For the boost converter, the output capacitor supplies
the load current when the main switch is on. The
required output capacitance is high, especially at high-
er duty cycles. Also, the output capacitor ESR needs to
be low enough to minimize the voltage drop due to the
ESR while supporting the load current. Use the follow-
ing equation to calculate the output capacitor for a
specified output ripple tolerance:
where I
PK
is the peak inductor current as defined in the
following
Power Dissipation
section, I
O
is the load cur-
rent, ΔV
Q
is the portion of the ripple due to the capaci-
tor 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 MAX5099 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 MAX5099 is available in a thermally enhanced pack-
age 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. The t
R
and t
F
can be measured in the actual
application.
The supply current in the MAX5099 is dependent on
the switching frequency. See the
Typical Operating
Characteristics
to find the supply current of the
MAX5099 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.
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
RMS
DC
PK
DC
PK
MAX
IN
O
O
I
=
+
+
Ч
(
)
(
)
Ч
=
Ч
2
2
3
N
N
L
IN
DS
SW
DC
IN
L
PK
IN
I
V
V
D
L f
I
I
I
I
I
Ч
=
(
)
Ч
Ч
=
=
−
−
η
Δ
Δ
2
++
=
×
ΔI
P
I
R
L
DC
RMS
ON MAX
2
2
(
)
I
I
I
I
I
I
P
V
I
t
t
f
DC
O
L
PK
O
L
SW
IN
O
R
F
S
=
= +
=
Ч Ч
+
(
)
Ч
−
Δ
Δ
2
2
W
W
4
I
I
I
I
I
D
P
I
R
RMS
DC
PK
DC
PK
MAX
DC
RMS
=
+
+
Ч
(
)
(
)
Ч
=
Ч
2
2
2
3
O
ON MAX
(
)
ESR
V
I
C
I
D
V
f
ESR
PK
OUT
O
MAX
Q
SW
=
=
Ч
Ч
Δ
Δ
ΔI
V
V
D
L
f
L
IN
DS
SW
=
(
)
Ч
Ч
−