LTC3444_15 Datasheet, PDF (16/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

LTC3444
U
OPERATIO
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 15ns) of the NMOS to
PMOS transition, improving efficiency. Use a surface
mount Schottky diode such as an MBRM120T3 or equiva-
lent. Do not use ordinary rectifier diodes, since the slow
recovery times will compromise efficiency.
Closing the Feedback Loop
The LTC3444 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck-boost), but is usually ~20dB. The
output filter exhibits a double pole response, as given by:
f _{FILTER_POLE} = _{2 • π •}
1
Hz
L • C_OUT
(in buck mode)
f _{FILTER_POLE}
=
2•
V_OUT
•
V_IN
π•
Hz
L • C_OUT
(in boost mode)
where L is in Henries and C_OUT is in farads.
The output filter zero is given by:
f
FILTER_ ZERO
=
2•
π
1
• R_ESR
•
C _OUT
Hz
where R_ESR is the equivalent series resistance of the
output cap.
C2
A troublesome problem when operating in boost mode is
dealing with the right-half plane zero (RHP), given by:
f _RHPZ
=
2•
π
V_IN2
• I_OUT •
L • V_OUT
Hz
The RHP zero has a +20dB/dec gain typical of a zero but
the –90° phase lag of a pole. This causes the loop gain to
flatten out while the phase margin decreases. The only way
to combat a RHP zero is to roll off the loop well before the
RHP zero frequency.
LOOP COMPENSATION NETWORKS
A simple Type I compensation network, refer to Figure 7,
can be incorporated to stabilize the loop, but at a cost of
reduced bandwidth and slower transient response. To
ensure proper phase margin using Type I compensation,
the loop must be crossed over at least a decade before the
output LC double pole frequency. The unity-gain fre-
quency of the error amplifier with the Type I compensation
is given by:
f
UG⁼
2•
π
1
Hz
• R1• C2
WCDMA applications demand an improved transient re-
sponse to the input control voltage. In other applications,
the output capacitor can be increased to meet help meet
the load transient requirements.
16
7 ^VC
FB
8
V_REF
R1
R2
V_OUT
3444 F07
Figure 7. Error Amplifier with Type I Compensation
3444fb