LT1507 Linear Technology, LT1507 Datasheet - Page 18

• Manufacturer Part Number: LT1507
• Description: 500kHz Monolithic Buck Mode Switching Regulator
• Manufacturer: Linear Technology

LT1507
APPLICATIONS
Example: with V
Total power dissipation is 0.3 + 0.046 + 0.032 = 0.38W.
Thermal resistance for the LT1507 packages is influenced
by the presence of internal or backside planes. With a full
plane under the SO package, thermal resistance will be
about 120 C/W. No plane will increase resistance to about
150 C/W. To calculate die temperature, use the proper
thermal resistance number for the desired package and
add in worst-case ambient temperature;
With the S8 package (
temperature of 70 C;
FREQUENCY COMPENSATION
The LT1507 uses a “current mode” architecture to help
alleviate phase shift created by the inductor. The basic
connections are shown in Figure 9. Gain of the power stage
can be modeled as 1.8A/V transconductance from the V
pin voltage to current delivered to the output. This is
shown in Figure 8 where the transconductance from V
pin to inductor current is essentially flat from 50Hz to
50kHz and phase shift is minimal in the important loop
unity-gain band of 1kHz to 50kHz. Inductor variation from
3 H to 20 H will have very little effect on these curves.
Overall gain from the V
the product of 1.8A/V transconductance multiplied by the
complex impedance of the load in parallel with the output
capacitor model.
The error amplifier can be modeled as a transconductance
of 2000 mho, with an output impedance of 200k
18
T
T
P
P
P
SW
Q
J
J
BOOST
= T
= 70 + 120(0.38) = 116 C
5 0 003
A
( .
( . )( ) ( . )
0 26 0 04 0 3
+
0 4 1 3 3
.
( . )
JA
3 3
(P
5
IN
5
)
2
TOT
2
= 5V, V
.
U
3 3 0 005
0 008
. ( .
)
.
C
pin to output is then modeled as
INFORMATION
JA
OUT
U
.
16 10
W
= 120 C/W) at an ambient
)
75
= 3.3V, I
1
0 032
.
9
W
0 046
.
OUT
1 5 500 10
W
W
= 1A;
U
3
in
C
C
parallel with 12pF. In all practical applications, the com-
pensation network from V
lower impedance than the output impedance of the ampli-
fier at frequencies above 500Hz. This means that the error
amplifier characteristics themselves do not contribute
excess phase shift to the loop and the phase/gain charac-
teristics of the error amplifier section are completely
controlled by the external compensation network.
The complete small-signal model is shown in Figure 9. R1
and R2 are the divider used to set output voltage. These are
internal on the fixed voltage LT1507-3.3 with R1 = 1.8k
and R2 = 5k. R
GND
Figure 9. Small-Signal Model for Loop Stability Analysis
12pF
200k
POWER STAGE
g
C
m
Figure 8. Phase and Gain from V
to Inductor Current
F
= 1.8A/V
2.0
1.5
1.0
0.5
0
R
ERROR AMPLIFIER
g
10
V
C
m
C
C
C
V
I
V
L = 10 H
= 2000 ho
OUT
OUT
IN
C
= 5V
, C
= 250mA
= 3.3V
100
C
and C
–
+
FREQUENCY (Hz)
LT1507
PHASE
2.42V
GAIN (A/V)
1k
C
F
pin to ground has a much
are external compensation
V
F
SW
B
10k
C
L1
LT1507 • F08
Pin Voltage
100k
R1
R2
80
40
0
–40
–80
1507 • F09
+
ESR
OUTPUT
C1