LTC3558_15 Datasheet, PDF (26/32 Pages) Linear Technology – Linear USB Battery Charger with Buck and Buck-Boost Regulators

LTC3558
APPLICATIONS INFORMATION
Buck-Boost Switching Regulator Output Voltage
Programming
The buck-boost switching regulator can be programmed
for output voltages greater than 2.75V and less than 5.45V.
To program the output voltage, a resistor divider is con-
nected between V_OUT2 and the feedback node (FB2) as
shown in Figure 9. The output voltage is given by V_OUT2
= 0.8(1 + R1/R2).
LTC3558
V_OUT2
R1
FB2
R2
3558 F09
Figure 9. Programming the Buck-Boost Output Voltage Requires
a Resistor Divider Connected Between V_OUT2 and FB2
Closing the Feedback Loop
The LTC3558 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,
boost, buck-boost), but is usually no greater than 20. The
output filter exhibits a double pole response given by:
f _{FILTER_POLE} = _{2 • π •}
1
Hz
L • C_OUT
where C_OUT is the output filter capacitor.
The output filter zero is given by:
f
FILTER
_
ZERO
=
2
•
π
•
1
R_ESR
•
C_OUT
Hz
where R_ESR is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-half
plane zero (RHP), and is given by:
f
RHPZ
=
2
•
π
•
PV_IN22
I_OUT • L
•
V_OUT2
Hz
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network, as shown in Figure
10, can be incorporated to stabilize the loop, but at the
cost of reduced bandwidth and slower transient response.
To ensure proper phase margin, the loop requires to be
crossed over a decade before the LC double pole.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
f
UG
=
2
•
π
•
1
R1•
C_P1
Hz
ERROR
AMP
0.8V
FB2
V_OUT2
R1
V_C2 ^CP1
R2
3558 F10
Figure 10. Error Amplifier with Type I Compensation
3558f
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