Data Sheet

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16
FAN3100C / FAN3100T — Single 2 A High-Speed, Low-Side Gate Driver
Operational Waveforms
At pow er up, the driver output remains LOW until the V
DD
voltage reaches the turn-on threshold. The magnitude of
the OUT pulses rises w ith V
DD
until steady-state V
DD
is
reached. The non-inverting operation illustrated in Figure
47 show s that the output remains LOW until the UVLO
threshold is reached, then the output is in-phase w ith the
input.
V
DD
IN+
IN-
OUT
Turn-on Threshold
Figure 47. Non-Inverting Start-Up Waveforms
For the inverting configuration of Figure 46, start-up
w aveforms are show n in Figure 48. With IN+ tied to V DD
and the input signal applied to IN–, the OUT pulses are
inverted w ith respect to the input. At pow er up, the
inverted output remains LOW until the V
DD
voltage
reaches the turn-on threshold, then it follow s the input
w ith inverted phase.
V
DD
IN+
(V
DD
)
IN-
OUT
Turn-on Threshold
Figure 48. Inverting Start-Up Waveforms
Thermal Guidelines
Gate drivers used to sw itch MOSFETs and IGBTs at high
frequencies can dissipate significant amounts of pow er. It
is important to determine the driver pow er dissipation and
the resulting junction temperature in the application to
ensure that the part is operating w ithin acceptable
temperature limits.
The total pow er dissipation in a gate driver is the sum of
tw o components; P
GAT E
and P
DYNAMIC
:
P
TOTAL
= P
GAT E
+ P
DYNAMIC
(1)
Gate Driving Loss: The most significant pow er loss
results from supplying gate current (charge per unit
time) to sw itch the load MOSFET on and off at the
sw itching frequency. The pow er dissipation that
results from driving a MOSFET at a specified gate-
source voltage, V
GS
, w ith gate charge, Q
G
, at
sw itching frequency, f
SW
, is determined by:
P
GAT E
= Q
G
• V
GS
• f
SW
(2)
Dynamic Pr e-drive / Shoot-through Current: A pow er
loss resulting from internal current consumption
under dynamic operating conditions, including pin
pull-up / pull-dow n resistors, can be obtained using
the I
DD
(no-Load) vs. Frequency graphs in Typical
Performance Characteristics to determine the current
I
DYNAMIC
draw n from V
DD
under actual operating
conditions:
P
DYNAMIC
= I
DYNA MIC
• V
DD
(3)
Once the pow er dissipated in the driver is determined, the
driver junction rise w ith respect to circuit board can be
evaluated using the follow ing thermal equation, assuming
ψ
JB
w as determined for a similar thermal design (heat
sinking and air flow ):
T
J
= P
TOTAL
•
ψ
JB
+ T
B
(4)
w here:
T
J
= driver junction temperature
ψ
JB
= (psi) thermal characterization parameter
relating temperature rise to total pow er
dissipation
T
B
= board temperature in location defined in the
Thermal Characteristics table.
In a typical forw ard converter application w ith 48 V input,
as show n in Figure 49, the FDS2672 w ould be a potential
MOSFET selection. The typical gate charge w ould be
32 nC w ith V
GS
= V
DD
= 10 V. Using a TTL input driver at a
sw itching frequency of 500 kHz, the total pow er
dissipation can be calculated as:
P
GAT E
= 32 nC • 10 V • 500 kHz = 0.160 W (5)
P
DYNAMIC
= 8 mA • 10 V = 0.080 W
(6)
P
TOTAL
= 0.24 W (7)
The 5-pin SOT23 has a junction-to-lead thermal
characterization parameter
ψ
JB
= 51°C/W.
In a system application, the localized temperature around
the device is a function of the layout and construction of
the PCB along w ith airflow across the surfaces. To
ensure reliable operation, the maximum junction
temperature of the device must be prevented from
exceeding the maximum rating of 150°C; w ith 80%
derating, T
J
w ould be limited to 120°C. Rearranging
Equation 4 determines the board temperature required to
maintain the junction temperature below 120°C:
T
B,MA X
= T
J
- P
TOTAL
•
ψ
JB
(8)
T
B,MA X
= 120°C – 0.24W • 51°C/W = 108°C (9)
For comparison purposes, replace the 5-pin SOT23 used
in the previous example w ith the 6-pin MLP package w ith
ψ
JB
= 2.8°C/W. The 6-pin MLP package can operate at a
PCB temperature of 119°C, w hile maintaining the junction
temperature below 120°C. This illustrates that the
physically smaller MLP package w ith thermal pad offers a
more conductive path to remove the heat from the driver.
Consider the tradeoffs betw een reducing overall circuit
size w ith junction temperature reduction for increased
reliability.