Datasheet

10
LT1310
sn1310 1310fs
APPLICATIO S I FOR ATIO
WUUU
Using the circuit of Figure 1 as an example, the following
table shows the parameters used to generate the Bode plot
shown in Figure 5.
Table 4. Bode Plot Parameters
PARAMETER VALUE UNITS COMMENT
R
L
30 Ω Application Specific
C
OUT
4.7 µF Application Specific
R
O
2MΩ Not Adjustable
C
C
820 pF Adjustable
R
C
15 kΩ Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
g
ma
500 µmho Not Adjustable
g
mp
1.5 mho Not Adjustable
L 5.6 µH Application Specific
f
S
1.6 MHz Adjustable
ESR 10 mΩ Not Adjustable
From Figure 5, the phase is 120° when the gain reaches
0dB giving a phase margin of 60°. This is more than
adequate. The crossover frequency is 50kHz, which is
about three times lower than the frequency of the right half
plane zero Z2. It is important that the crossover frequency
be at least three times lower than the frequency of the RHP
zero to achieve adequate phase margin.
–
+
–
+
g
ma
R
C
R
O
R2
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
1310 F04
R1
C
OUT
FB
R
L
V
OUT
V
C
C
C
g
mp
1.255V
REFERENCE
Figure 4. Boost Converter Equivalent Model
Diode Selection
A Schottky diode is recommended for use with the LT1310.
The Microsemi UPS120 is a very good choice. Where the
input to output voltage differential exceeds 20V, use the
UPS140 (a 40V diode). These diodes are rated to handle an
average forward current of 1A. For applications where the
average forward current of the diode is less than 0.5A, an
ON Semiconductor MBR0520 diode can be used.
Setting Output Voltage
To set the output voltage, select the values of R1 and R2
(see Figure 1) according to the following equation:
RR
V
V
OUT
12
1 255
1=






.
–
A good range for R2 is from 5k to 30k.
FREQUENCY (Hz)
0
GAIN (dB)
50
100
100 10k 100k 1M
1310 F05a
–50
1k
FREQUENCY (Hz)
PHASE (DEG)
–100
0
100 10k 100k 1M
1946 F05b
–200
–180
1k
60°
Figure 5. Bode Plot of Figure 1’s Circuit