Datasheet

LM5088, LM5088-Q1
www.ti.com SNVS600H –DECEMBER 2008–REVISED MARCH 2013
The cool down time (t2) is set by the time taken to discharge the RES cap with 1.2 µA current source. This
feature will reduce the input power drawn by the converter during a prolonged over current condition. In this
application 500 µs of delay time was selected. The minimum value of C
RES
capacitor should be no less than
0.022 µF.
MOSFET SELECTION
Selection of the Buck MOSFET is governed by the same tradeoffs as the switching frequency. Losses in power
MOSFETs can be broken down into conduction losses and switching losses. The conduction loss is given by:
P
DC
= D x (I
O
2
x R
DS(ON)
x 1.3)
where
• D is the duty cycle
• IO is the maximum load current (24)
The factor 1.3 accounts for the increase in MOSFET on-resistance due to heating. Alternatively, for a more
precise calculation, the factor of 1.3 can be ignored and the on-resistance of the MOSFET can be estimated
using the R
DS(ON)
vs. Temperature curves in the MOSFET datasheet.
The switching loss occurs during the brief transition period as the MOSFET turns on and off. During the transition
period both current and voltage are present in the MOSFET. The switching loss can be approximated as:
P
SW
= 0.5 x V
IN
x I
O
x (t
R
+ t
F
) x f
SW
where
• t
R
the rise time of the MOSFET
• t
F
is the fall time of the MOSFET (25)
The rise and fall times are usually mentioned in the MOSFET datasheet or can be empirically observed on the
scope. Another loss, which is associated with the buck MOSFET is the “gate-charging loss”. This loss differs
from the above two losses in the sense that it is dissipated in the LM5088 and not in the MOSFET itself. Gate
charging loss, P
GC
, results from the current driving charging the gate capacitance of the power MOSFETs and is
approximated as:
P
GC
= VCC x Q
g
x f
SW
(26)
For this example with the maximum input voltage of 55V, the Vds breakdown rating of the selected MOSFET
must be greater than 55V plus any ringing across drain to source due to parasitics. In order to minimize switching
time and gate drive losses, the selected MOSFET must also have low gate charge (Q
g
). A good choice of
MOSFET for this design example is the SI7148DP which has a total gate charge of 30nC and rise and fall times
of 10 ns and 12 ns respectively.
DIODE SELECTION
A Schottky type re-circulating diode is required for all LM5088 applications. The near ideal reverse recovery
current transients and low forward voltage drop are particularly important diode characteristics for high input
voltage and low output voltage applications common to LM5088. The diode switching loss is minimized in a
Schottky diode because of near ideal reverse recovery. The conduction loss can be approximated by:
P
dc_diode
= (1 - D) x I
O
x V
F
where
• V
F
is the forward drop of the diode (27)
The worst case is to assume a short circuit load condition. In this case, the diode will carry the output current
almost continuously. The reverse breakdown rating should be selected for the maximum input voltage level plus
some additional safety margin to withstand ringing at the SW node. For this application a 60V On Semiconductor
Schottky diode (MBRB2060) with a specified forward drop of 0.6V at 7A at a junction temperature of 50°C was
selected. For output loads of 5A and greater and high input voltage applications, a diode in a D
2
PAK package is
recommended to support the worst case power dissipation
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