Datasheet

LM25088
LM25088-Q1
www.ti.com SNVS609H –DECEMBER 2008–REVISED MARCH 2013
For this example with the maximum input voltage of 36V, the Vds breakdown rating of the selected MOSFET
must be greater than 36V 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 SI7848DP 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 LM25088 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 LM25088. 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 (26)
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 45V On Semiconductor
Schottky diode (MBRB1545) with a specified forward drop of 0.5V 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
SNUBBER COMPONENTS SELECTION
Excessive ringing and spikes can cause erratic operation and couple spikes and noise to the output. Voltage
spikes beyond the rating of the LM25088 or the re-circulating diode can damage these devices. A snubber
network across the power diode reduces ringing and spikes at the switching node. Selecting the values for the
snubber is best accomplished through empirical methods. First, make sure that the lead lengths for the snubber
connections are very short. For the current levels typical for the LM25088, a resistor value between 3 and 10Ω
should be adequate. As a rule of thumb, a snubber capacitor which is 4~5 times the Schottky diode’s junction
capacitance will reduce spikes adequately. Increasing the value of the snubber capacitor will result in more
damping but also results in higher losses. The resistor’s power dissipation is independent of the resistance value
as the resistor dissipates the energy stored by the snubber capacitor. The resistor’s power dissipation can be
approximated as:
P
R_SNUB
= C
SNUB
x VIN
max
2
x f
SW
(27)
ERROR AMPLIFIER COMPENSATION
R
COMP
, C
COMP
and C
HF
configure the error amplifier gain characteristics to accomplish a stable voltage loop gain.
One advantage of current mode control is the ability of to close the loop with only two feedback components
R
COMP
and C
COMP
. The voltage loop gain is the product of the modulator gain and the error amplifier gain. For
this example, the modulator can be treated as an ideal voltage-to-current (transconductance) converter, The DC
modulator gain of the LM25088 can be modeled as:
DC Gain
(MOD)
= R
LOAD
/ (A x R
S
) (28)
The dominant low frequency pole of the modulator is determined by the load resistance (R
LOAD
) and the output
capacitance (C
OUT
). The corner frequency of this pole is:
For, R
LOAD
= 5V/7A = 0.714Ω and C
OUT
= 500 µF (effective), then FP
(MOD)
= 550 Hz.
DC Gain
(MOD)
= 0.714/ (10 x 10 mΩ) = 7.14 = 17dB (29)
For the 5V design example the modulator gain vs. frequency characteristic was measured as shown in Figure 25.
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