LM5111-2M National Semiconductor, LM5111-2M Datasheet - Page 8

• Manufacturer Part Number: LM5111-2M
• Description: Dual 5A Compound Gate Driver
• Manufacturer: National Semiconductor

www.national.com
Layout Considerations
Attention must be given to board layout when using LM5111.
Some important considerations include:
1. A Low ESR/ESL capacitor must be connected close to
2. Proper grounding is crucial. The drivers need a very low
3. With the rise and fall times in the range of 10 ns to 30 ns,
4. The LM5111 SOIC footprint is compatible with other
5. If either channel is not being used, the respective input
Thermal Performance
INTRODUCTION
The primary goal of thermal management is to maintain the
integrated circuit (IC) junction temperature (T
specified maximum operating temperature to ensure reliabil-
ity. It is essential to estimate the maximum T
nents in worst case operating conditions. The junction tem-
perature is estimated based on the power dissipated in the
IC and the junction to ambient thermal resistance θ
IC package in the application board and environment. The
θ
the printed circuit board design and the operating environ-
ment.
DRIVE POWER REQUIREMENT CALCULATIONS IN
LM5111
The LM5111 dual low side MOSFET driver is capable of
sourcing/sinking 3A/5A peak currents for short intervals to
drive a MOSFET without exceeding package power dissipa-
tion limits. High peak currents are required to switch the
MOSFET gate very quickly for operation at high frequencies.
JA
is not a given constant for the package and depends on
the IC and between the V
high peak currents being drawn from V
of the MOSFET.
impedance path for current return to ground avoiding
inductive loops. The two paths for returning current to
ground are a) between LM5111 V
of the circuit that controls the driver inputs, b) between
LM5111 V
being driven. All these paths should be as short as
possible to reduce inductance and be as wide as pos-
sible to reduce resistance. All these ground paths should
be kept distinctly separate to avoid coupling between the
high current output paths and the logic signals that drive
the LM5111. A good method is to dedicate one copper
plane in a multi-layered PCB to provide a common
ground surface.
care is required to minimize the lengths of current car-
rying conductors to reduce their inductance and EMI
from the high di/dt transients generated by the LM5111.
industry standard drivers including the TC4426/27/28
and UCC27323/4/5.
pin (IN_A or IN_B) should be connected to either V
V
CC
to avoid spurious output signals.
EE
pin and the source of the power MOSFET
CC
and V
EE
pin and the ground
EE
CC
pins to support
J
during turn-on
of IC compo-
J
) below a
JA
for the
EE
or
8
The schematic above shows a conceptual diagram of the
LM5111 output and MOSFET load. Q1 and Q2 are the
switches within the gate driver. R
the external MOSFET, and C
tance of the MOSFET. The gate resistance Rg is usually very
small and losses in it can be neglected. The equivalent gate
capacitance is a difficult parameter to measure since it is the
combination of C
(gate to drain capacitance). Both of these MOSFET capaci-
tances are not constants and vary with the gate and drain
voltage. The better way of quantifying gate capacitance is
the total gate charge Q
charge required by C
voltage V
Assuming negligible gate resistance, the total power dissi-
pated in the MOSFET driver due to gate charge is approxi-
mated by
Where
F
For example, consider the MOSFET MTD6N15 whose gate
charge specified as 30 nC for V
The power dissipation in the driver due to charging and
discharging of MOSFET gate capacitances at switching fre-
quency of 300 kHz and V
If both channels of the LM5111 are operating at equal fre-
quency with equivalent loads, the total losses will be twice as
this value which is 0.216W.
In addition to the above gate charge power dissipation, -
transient power is dissipated in the driver during output
transitions. When either output of the LM5111 changes state,
current will flow from V
time through the output totem-pole N and P channel
MOSFETs. The final component of power dissipation in the
driver is the power associated with the quiescent bias cur-
rent consumed by the driver input stage and Under-voltage
lockout sections.
Characterization of the LM5111 provides accurate estimates
of the transient and quiescent power dissipation compo-
nents. At 300 kHz switching frequency and 30 nC load used
in the example, the transient power will be 8 mW. The 1 mA
nominal quiescent current and 12V V
12 mW typical quiescent power.
Therefore the total power dissipation
SW
= switching frequency of the MOSFET.
P
DRIVER
GATE
P
D
= 0.216 + 0.008 + 0.012 = 0.236W.
.
P
= 12V x 30 nC x 300 kHz = 0.108W.
DRIVER
GS
(gate to source capacitance) and C
GS
CC
= V
FIGURE 2.
G
GATE
and C
to V
in coloumbs. Q
GATE
IN
is the equivalent gate capaci-
EE
of 12V is equal to
GATE
GD
x Q
G
for a very brief interval of
is the gate resistance of
G
for a given gate drive
GATE
= 12V.
x F
SW
supply produce a
G
combines the
20112307
GD