LTC3424_15 Datasheet, PDF (10/12 Pages) Linear Technology – Low Output Voltage, 3MHz Micropower Synchronous Boost Converters

LTC3423/LTC3424
APPLICATIO S I FOR ATIO
G_DC = G_{CONTROLOUTPUT} • G_EA
G_CONTROL
=
2 • V_IN
I_OUT
,
G_EA
≈
2000
The output filter pole is given by:
f_FILTERPOLE
=
π
I_OUT
• V_OUT • C_OUT
Hz
where C_OUT is the output filter capacitor.
The output filter zero is given by:
f_FILTERZERO
=
2
•
π
•
1
R_ESR
•
C_OUT
Hz
where R_ESR is the capacitor equivalent series resistance.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP) and is given by:
f_RHPZ =
V_IN2 •R_{O Hz}
2
2 • π • L • V_O
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
rolled off before the RHP zero frequency.
The typical error amp compensation is shown in Figure 3.
The equations for the loop dynamics are as follows:
f_POLE1
≈
2•
π
•
1
20 •10⁶
Hz
• C_C1
which is extremelyclose to DC
f_ZERO1
=
2•
π
1
• R_Z
Hz
• C_C1
f_POLE2
≈
2
•
π
1
• R_Z
• C_C2
Hz
Refer to Application Note AN76 for more closed loop
examples.
ERROR
AMP
1.25V
FB
8
V_C
9
C_C1
R_Z
Figure 3
V_OUT
R1
R2
C_C2
3423/24 F03
TYPICAL APPLICATIO
Typical Application with Output Disconnect
V_IN = 0.9V TO 1.5V
ZETEX
FMMT717
LTC3423/LTC3424
3
4
V_IN
SW
RB*
10
SHDN
7
V_OUT
2
8
MODE/SYNC FB
V_DD ⁶ _VDD
1
R_t
9
V_C
5
GND
V_OUT
C5
1µF
10
*
SET
RB
TO
FORCE
BETA
OF
≤ 100;
RB
=
(V_OUT
–
V_INMIN – 0.7V)
I_OUTMAX
•
100
0 = FIXED FREQ
1 = Burst Mode OPERATION
3423/24 TA03
34234f