LT3436 LINER [Linear Technology], LT3436 Datasheet - Page 11

• Manufacturer Part Number: LT3436
• Description: 3A, 800kHz Step-Up Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
The inductor must have a rating greater than its peak
operating current to prevent saturation resulting in effi-
ciency loss. Peak inductor current is given by:
Also, consideration should be given to the DC resistance
of the inductor. Inductor resistance contributes directly to
the efficiency losses in the overall converter.
THERMAL CALCULATIONS
Power dissipation in the LT3436 chip comes from four
sources: switch DC loss, switch AC loss, drive current, and
input quiescent current. The following formulas show how
to calculate each of these losses. These formulas assume
continuous mode operation, so they should not be used
for calculating efficiency at light load currents.
Switch loss:
R
Example: V
Total power dissipation = 0.34 + 0.31 + 0.11 + 0.005 =
0.77W
Thermal resistance for LT3436 package is influenced by
the presence of internal or backside planes. With a full
plane under the package, thermal resistance will be about
40°C/W. To calculate die temperature, use the appropriate
V
SW
IN
I
DC duty cycle
I
P
P
LPEAK
SW
SW
VIN
loss:
= Switch resistance (≈ 0.16Ω hot)
,
=
=
=
(
=
(
V
DC I
(
V
IN
OUT
IN SW
(
V
= 5V, V
)(
OUT
)(
V
V
)(
IN
I
SW
IN
50
I
OUT
)(
=
•
U
) (
)(
I
2
OUT
η
(
OUT
DC
V
)
R
OUT
SW
)
)
V
INFORMATION
= 12V and I
U
+
OUT
+
)
−
1
+
V V
mA V
IN OUT
V
17
2
IN
(
V
(
OUT
n I
)
( )
IN
SW
W
)
( )( )
OUT
L f
−
(
V
V
IN
= 0.8A
OUT
)
)
U
( )
f
thermal resistance number and add in worst-case ambient
temperature:
If a true die temperature is required, a measurement of
the SYNC to GND pin resistance can be used. The SYNC
pin resistance across temperature must first be cali-
brated, with no device power, in an oven. The same
measurement can then be used in operation to indicate the
die temperature.
FREQUENCY COMPENSATION
Loop frequency compensation is performed on the output
of the error amplifier (V
The main pole is formed by the series capacitor and the
output impedance (≈500kΩ) of the error amplifier. The
pole falls in the range of 2Hz to 20Hz. The series resistor
creates a “zero” at 1kHz to 5kHz, which improves loop
stability and transient response. A second capacitor, typi-
cally one-tenth the size of the main compensation capaci-
tor, is sometimes used to reduce the switching frequency
ripple on the V
voltage ripple attenuated by the output divider and multi-
plied by the error amplifier. Without the second capacitor,
V
To prevent irregular switching, V
kept below 50mV
maximum output load current and will also be increased if
poor quality (high ESR) output capacitors are used. The
addition of a 150pF capacitor on the V
switching frequency ripple to only a few millivolts. A low
value for R
margin may be inadequate.
C
T
V
V
g
R
V
pin ripple is:
J
m
C
RIPPLE
OUT
C
= T
Pin Ripple =
= Series resistor on V
= Error amplifier transconductance
(≈850µmho)
A
= DC output voltage
+ θ
C
= Output ripple (V
will also reduce V
JA
C
(P
pin. V
P–P
TOT
1.2(V
. Worst-case V
)
C
C
RIPPLE
pin ripple is caused by output
pin) with a series RC network.
(V
C
C
OUT
P–P
pin
pin ripple, but loop phase
)(g
)
C
)
m
C
pin ripple should be
)(R
pin ripple occurs at
C
)
C
LT3436
pin reduces
11
3436fa