LT1310_15 Datasheet, PDF (10/12 Pages) Linear Technology – 1.5A Boost DC/DC Converter with Phase-Locked Loop

LT1310
APPLICATIO S I FOR ATIO
–
g_mp
+
V_OUT
C_OUT R_L
V_C
R_C R_O
1.255V
REFERENCE
g_ma
R1
FB
C_C
R2
1310 F04
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
Figure 4. Boost Converter Equivalent Model
100
50
0
–50
100
0
1k
10k
100k
FREQUENCY (Hz)
1M
1310 F05a
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
R_L
30
Ω
C_OUT
4.7
µF
R_O
2
MΩ
C_C
820
pF
R_C
15
kΩ
V_OUT
12
V
V_IN
5
V
g_ma
500
µmho
g_mp
1.5
mho
L
5.6
µH
f_S
1.6
MHz
ESR
10
mΩ
COMMENT
Application Specific
Application Specific
Not Adjustable
Adjustable
Adjustable
Application Specific
Application Specific
Not Adjustable
Not Adjustable
Application Specific
Adjustable
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.
10
–100
–180
–200
100
60°
1k
10k
100k
FREQUENCY (Hz)
1M
1946 F05b
Figure 5. Bode Plot of Figure 1’s Circuit
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:
R1
=
R2^{}
V_OUT
1.255V
–
1^{}
A good range for R2 is from 5k to 30k.
sn1310 1310fs