LT3507A
22
3507afa
For more information www.linear.com/LT3507A
The maximum allowed power dissipation by the LT3507A
can be determined by:
P
DISS(MAX)
=
T
J(MAX)
– T
A
θ
where T
JMAX
is the maximum die temperature of 125°C
(150°C for H-grade).
However, take care in determining T
A
since the catch di-
odes also dissipate power and must be located close to
the LT3507A. Another potential heat sour
ce is the LDO
pass transistor. In a compact layout the pass transistor
will be located close to the LT3507A. The inductors will
also dissipate some power due to their series resistance
and they must be close to the LT3507A. All of these heat
sources will increase the effective ambient temperature
seen by the LT3507A.
A thorough analysis of eight heat sources in a small PCB
area is beyond the scope of this data sheet, however a
number of thermal analysis programs are available to
calculate the temperature rise in each component (such as
PCAnalyze from K&K Associates or Flo Therm PCB from
Mentor). The power dissipation of each component will be
needed to accurately calculate the thermal characteristics
of the system.
The contributors to power dissipation inside the LT3507A
are switch DC loss, switch AC loss, boost current, quies
-
cent current and LDO drive current. The total dissipation
within the LT3507A can be expressed as:
P
DISS
= P
SWDCi
+P
SWACi
+P
BSTi
( )
+P
Q
+P
LDO
3
∑
The switch DC and AC losses in channel i are:
P
SWDCi
=
R
SWi
I
OUTi
( )
2
V
OUTi
V
INi
P
SWACi
= 17ns I
OUTi
V
INi
( )
f
( )
applications inForMation
where R
SWi
is the equivalent switch resistance (0.18Ω for
channel 1 and 0.22Ω for channels 2 and 3) and f is the
operating frequency.
The boost loss in channel i is:
P
BSTi
=
V
OUTi
V
BOOSTi
( )
I
OUTi
50
+0.02A
⎛
⎝
⎜
⎞
⎠
⎟
V
The quiescent loss is:
P
Q
= V
IN1
(I
Q(VIN1)
) + V
BIAS
(I
Q(BIAS)
)
If the BIAS pin does not have a voltage of at least 3V ap-
plied, then V
IN1
must replace V
BIAS
in the equation. Also,
I
Q(VIN1)
can be reduced by 0.2mA (typ) if the LDO is shut
off (see the LDO section).
The LDO drive loss is:
P
LDO
=(V
BIAS
−V
LDO(OUT)
−0.7V)
I
OUT(LDO)
β
PASS
⎛
⎝
⎜
⎞
⎠
⎟
,
if V
BIAS
≥ V
LDO(OUT)
+1.5V
or
P
LDO
=(V
IN1
−V
LDO(OUT)
−0.7V)
I
OUT(LDO)
β
PASS
⎛
⎝
⎜
⎞
⎠
⎟
,
if V
BIAS
<V
LDO(OUT)
+1.5V
where β
PASS
is the current gain of the external pass
transistor.
Next, the power in the external components must be taken
into account. The diode power is given by:
P
DIODE
=
V
F
V
IN
– V
OUT
– V
F
I
OUT
V
where V
F
is the forward drop of the diode at I
OUT
.
The inductor power is:
P
IND
= (I
OUT
)
2
ESR
IND
where ESR
IND
is the inductor equivalent series resistance.
