Datasheet

MAX5088/MAX5089
rent through the internal power MOSFET (P
MOSFET
).
The total power dissipated in the package must be lim-
ited so the junction temperature does not exceed its
absolute maximum rating of +150°C at maximum ambi-
ent temperature. Calculate the power lost in the
MAX5088/MAX5089 using the following equations:
The power dissipated in the switch is:
P
MOSFET
= I
RMS_MOSFET
x R
ON
where:
∆I
P-P
is the peak-to-peak inductor current ripple.
The power lost due to switching the internal power
MOSFET is:
t
R
and t
F
are the rise and fall times of the internal power
MOSFET measured at SOURCE.
The power lost due to the switching quiescent current
of the device is:
P
Q
= V
IN
x I
SW
(MAX5088)
The switching quiescent current (I
SW
) of the
MAX5088/MAX5089 is dependent on switching fre-
quency. See the Typical Operating Characteristics sec-
tion for the value of I
SW
at a given frequency.
In the case of the MAX5089, the switching current
includes the synchronous rectifier MOSFET gate-drive
current (I
SW-DL
). The I
SW-DL
depends on the total gate
charge (Q
g-DL
) of the synchronous rectifier MOSFET
and the switching frequency.
P
Q
= V
IN
x (I
SW
+ I
SW-DL
) (MAX5089)
I
SW-DL
= Q
g-DL
x f
SW
where the Q
g-DL
is the total gate charge of the synchro-
nous rectifier MOSFET at V
GS
= 5V.
The total power dissipated in the device is:
P
TOTAL
= P
MOSFET
+ P
SW
+ P
Q
Calculate the temperature rise of the die using the fol-
lowing equation:
T
J
= T
C
+ (P
TOTAL
x θ
JC
)
θ
JC
is the junction-to-case thermal resistance equal to
1.7°C/W. T
C
is the temperature of the case and T
J
is
the junction temperature, or die temperature. The case-
to-ambient thermal resistance is dependent on how
well heat can be transferred from the PC board to the
air. Solder the underside exposed pad to a large cop-
per GND plane. If the die temperature reaches +170°C
the MAX5088/MAX5089 shut down and do not restart
again until the die temperature cools by 25°C.
Compensation
The MAX5088/MAX5089 have an internal transconduc-
tance error amplifier with an inverting input (FB) and
output (COMP) available for external frequency com-
pensation. The flexibility of external compensation and
high switching frequencies for the MAX5088/MAX5089
allow a wide selection of output filtering components,
especially the output capacitor. For cost-sensitive
applications, use high-ESR aluminum electrolytic
capacitors. For size sensitive applications, use low-ESR
tantalum or ceramic capacitors at the output.
Before designing the compensation components, first
choose all the passive power components that meet
the output ripple, component size, and component cost
requirements. Secondly, choose the compensation
components to achieve the desired closed-loop band-
width and phase margin. Use a simple 1-zero, 2-pole
pair (Type II) compensation if the output capacitor ESR
zero frequency (f
ZESR
) is below the unity-gain
crossover frequency (f
C
). Use a 2-zero, 2-pole (Type
III) compensation when the f
ZESR
is higher than f
C
.
Use procedure 1 to calculate the compensation net-
work components when f
ZESR
< f
C
.
Procedure 1 (see Figure 3)
Calculate the f
ZESR
and f
LC
double pole:
Calculate the unity-gain crossover frequency as:
If f
ZESR
is lower than f
C
and close to f
LC
, use a Type II
compensation network where R
F
C
F
provides a midband
zero (f
mid,zero
) and R
F
C
CF
provides a high-frequency pole.
f
f
C
SW
=
20
f
ESR C
f
LC
ZESR
OUT
LC
OUT
=
××
=
××
1
2
1
2
π
π
P
VI tt f
SW
IN OUT R F SW
=
××+
()
×
4
IID
ID
RMS MOSFET OUT
PP
_
()=×+
×








−
2
2
12
∆
2.2MHz, 2A Buck Converters with an
Integrated High-Side Switch
16 ______________________________________________________________________________________