Datasheet

LTC3775
17
3775fa
The value of Q
G
should come from the plot of V
GS
vs
Q
G
in the Typical Performance Characteristics section of
the MOSFET data sheet. The value listed in the electrical
specifi cations may be measured at a higher V
GS
, such
as 10V, whereas the value of interest is at the 5V INTV
CC
gate drive voltage.
Care must be taken to ensure that the maximum junction
temperature of the LTC3775 is never exceeded. The junc-
tion temperature can be estimated using the following
equations:
P
DISS
= V
IN
• I
INTVCC
T
J
= T
A
+ P
DISS
• R
TH(JA)
As an example of the required thermal analysis, consider
a buck converter with a 24V input voltage and an output
voltage of 3.3V at 15A. The switching frequency is 500kHz
and the maximum ambient temperature is 70°C. The power
MOSFET used for this application is the Vishay Siliconix
Si7884DP, which has a typical R
DS(ON)
of 7.5m at V
GS
= 4.5V and 5.5m at V
GS
= 10V. From the plot of V
GS
vs
Q
G
, the total gate charge at V
GS
= 5V is 18.5nC (the tem-
perature coeffi cient of the gate charge is low). One power
MOSFET is used for the top side and one for the bottom
side. For the UD package:
I
INTVCC
= 3.5mA + 2 • 18.5nC • 500kHz = 22mA
P
DISS
= 24V • 22mA = 528mW
T
J
= 70°C + 528mW • 68°C/W = 105.9°C
In this example, the junction temperature rise is 35.9°C.
These equations demonstrate how the gate charge cur-
rent typically dominates the quiescent current of the IC,
and how the choice of the operating frequency and board
heat sinking can have a signifi cant effect on the thermal
performance of the solution.
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current of the IC should
be checked when operating in continuous mode (heavy
load) at maximum V
IN
. A trade-off between the operat-
ing frequency and the size of the power MOSFETs may
need to be made in order to maintain a reliable junction
temperature.
Finally, it is important to verify the calculations by perform-
ing a thermal analysis of the fi nal PCB using an infrared
camera or thermal probe.
Operation at Low Supply Voltage
The LTC3775 has a minimum input voltage of 4.5V. The
gate driver for the LTC3775 consists of a PMOS pull-up
and an NMOS pull-down device, allowing the full INTV
CC
voltage to be applied to the gates during power MOSFET
switching. Nonetheless, care should be taken to deter-
mine the minimum gate drive supply voltage (INTV
CC
) in
order to choose the optimum power MOSFETs. Important
parameters that can affect the minimum gate drive volt-
age are the minimum input voltage (V
IN(MIN)
), the LDO
dropout voltage, the Q
G
of the power MOSFETs, and the
operating frequency.
If the input voltage V
IN
is low enough for the INTV
CC
LDO
to be in dropout, then the minimum gate drive supply
voltage is:
V
INTVCC
= V
IN(MIN)
– V
DROPOUT
The LDO dropout voltage is a function of the total gate
drive current and the quiescent current of the IC (typically
3.5mA). A curve of dropout voltage versus output cur-
rent for the LDO is shown in Figure 10. The temperature
coeffi cient of the LDO dropout voltage is approximately
6000ppm/°C. See the INTV
CC
Regulator and Thermal
Considerations sections for information about calculating
the total quiescent current.
APPLICATIONS INFORMATION
Figure 10. INTV
CC
LDO Dropout Voltage vs Current
INTV
CC
LOAD CURRENT (mA)
0
INTV
CC
DROPOUT VOLTAGE (V)
–0.4
–0.2
0
40
3775 F10
–0.6
–0.8
–1.0
10
20
30
50
T
A
= 25°C