LT1956IGN-5 Linear Technology, LT1956IGN-5 Datasheet - Page 19

• Manufacturer Part Number: LT1956IGN-5
• Description: IC SW REG STP-DWN HI VOLT 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1956IGN-5

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LT1956IGN5

APPLICATIO S I FOR ATIO
Boost current loss:
Quiescent current loss:
Example: with V
Total power dissipation in the IC is given by:
Thermal resistance for the LT1956 packages is influenced
by the presence of internal or backside planes.
SSOP (GN16) Package: With a full plane under the GN16
package, thermal resistance will be about 85 C/W.
TSSOP (Exposed Pad) Package: With a full plane under the
TSSOP package, thermal resistance (
45 C/W.
To calculate die temperature, use the proper thermal
resistance (
worst-case ambient temperature:
When estimating ambient, remember the nearby catch
diode and inductor will also be dissipating power.
P
P
P
SW
Q
BOOST
R
t
t
t
t
f = switch frequency
P
T
P
P
EFF
r
f
Ir
J
BOOST
Q
TOT
SW
= (V
= (V
= t
12 0 0015
= T
= effective switch current/voltage overlap time
= (t
0 125 0 171 0 296
= switch resistance ( 0.3) hot
If
= 0.296W + 0.058W + 0.033W = 0.39W
= P
V
0 3 1 5
.
A
IN
IN
IN
= (I
.
r
.
+ (
/1.7)ns
/1.2)ns
5
SW
+ t
12
0 0015
JA
V
OUT
2
.
OUT
f
JA
12
+ P
) number for the desired package an add in
2
+ t
1 36
IN
/0.05)ns
.
/
• P
2
Ir
BOOST
= 12V, V
U
V
5 0 003
I
+ t
TOT
OUT
IN
If
57 10
V
.
)
)
OUT
0 058
/
+ P
36
.
U
•
.
OUT
Q
0 003
W
W
.
9
= 5V and I
0 033
.
1 2 1 12 500 10
W
/
W
JA
OUT
) will be about
= 1A:
U
•
3
Notice that the catch diode’s forward voltage contributes
a significant loss in the overall system efficiency. A larger,
low V
Typical thermal resistance of the board is 10 C/W. Taking
the catch diode and inductor power dissipation into ac-
count and using the example calculations for LT1956 dis-
sipation, the LT1956 die temperature will be estimated as:
With the GN16 package (
temperature of 70 C:
With the TSSOP package (
temperature of 70 C:
Die temperature can peak for certain combinations of
V
switch AC losses, quiescent and catch diode losses, a
lower V
losses. In general, the maximum and minimum V
should be checked with maximum typical load current for
calculation of the LT1956 die temperature. If a more
accurate die temperature is required, a measurement of
the SYNC pin resistance (to GND) can be used. The SYNC
pin resistance can be measured by forcing a voltage no
greater than 0.5V at the pin and monitoring the pin
current over temperature in a oven. This should be done
with minimal device power (low V
[V
ambient (oven) temperature.
IN
C
V
P
L
P
T
T
T
P
P
, V
DCR
J
J
J
= 0V]) in order to calibrate SYNC pin resistance with
DIODE
F
DIODE
INDUCTOR
INDUCTOR
F
= T
= 70 + (85 • 0.39) + (10 • 0.47) = 108 C
= 70 + (45 • 0.37) + (10 • 0.47) = 91 C
= Forward voltage of diode (assume 0.63V at 1A)
OUT
diode can improve efficiency by several percent.
IN
= inductor DC resistance (assume 0.1 )
A
and load current. While higher V
+ (
may generate greater losses due to switch DC
( )(
( . )(
0 63 12 5 1
V V
= (I
= (1)(0.1) = 0.1W
JA
F
LOAD
• P
IN
12
TOT
–
)(L
V
– )( )
) + (10 • [P
V
IN
DCR
OUT
LT1956/LT1956-5
JA
JA
)
)(
= 85 C/W), at an ambient
I
LOAD
= 45 C/W) at an ambient
0 37
.
DIODE
IN
)
W
and no switching
+ P
IN
gives greater
INDUCTOR
IN
19
levels
])
1956f