LTC3122_15 Datasheet, PDF (16/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3122
Applications Information
The compensation will force the converter gain G_BOOST
to unity at ƒ_C by using the following expression for C_C:
C_C
=
10³
• g_ma • R2 • G_ƒC (a₁
2π • ƒ_C • (R1+ R2)
− 1)
a₁
a_{2 pF}
(g_ma in µS, ƒ_C in kHz, G_ƒC in V/V)
Once C_C is calculated, R_C and C_F are determined by:
R_C
=
10⁶ •
2π • ƒ_C
a₁
• C_C
kΩ
(ƒ_C
in
kHz,
C_C
in
pF)
C_F
=
C_C
a₁ − 1
The values of the phase lead components are given by
the expressions:
R_PL
=
R1−
a₂
•
⎛
⎝⎜
R1• R2
R1+ R2
⎞
⎠⎟
kΩ
a₂ −1
and
C_PL
=
10⁶ ₍a₂ − 1₎₍R1+R2₎
2π • ƒ_C • R1² a₂
pF
where R1, R2, and R_PL are in kΩ and ƒ_C is in kHz.
Note that selecting Φ₂ = 0° forces a₂ = 1, and so the
converter will have Type II compensation and therefore
no feedforward: R_PL is open (infinite impedance) and C_PL
= 0pF. If a₂ = 0.833 • V_OUT (its maximum), feedforward is
maximized; R_PL = 0 and C_PL is maximized for this com-
pensation method.
Once the compensation values have been calculated, ob-
taining a converter bode plot is strongly recommended to
verify calculations and adjust values as required.
Using the circuit in Figure 5 as an example, Table 3 shows
the parameters used to generate the bode plot shown in
Figure 6.
Table 3. Bode Plot Parameters for Type II Compensation
PARAMETER
VALUE
UNITS
COMMENT
V_IN
5
V
App Specific
V_OUT
12
V
App Specific
R_L
15
Ω
App Specific
C_OUT
22
µF
App Specific
R_ESR
5
mΩ
App Specific
L
3.3
µH
App Specific
ƒ_OSC
1
MHz
Adjustable
R1
1020
kΩ
Adjustable
R2
113
kΩ
Adjustable
g_ma
95
µS
Fixed
R_O
10
MΩ
Fixed
g_mp
3.4
S
Fixed
η
80
%
App Specific
R_C
210
kΩ
Adjustable
C_C
390
pF
Adjustable
C_F
10
pF
Adjustable
R_PL
0
kΩ
Optional
C_PL
0
pF
Optional
From Figure 6, the phase is 60° when the gain reaches
0dB, so the phase margin of the converter is 60°. The
crossover frequency is 15kHz, which is more than three
times lower than the 108.4kHz frequency of the RHP zero
to achieve adequate phase margin.
3122fa
16
For more information www.linear.com/LTC3122