LTC3567 Datasheet, PDF (24/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter with I2C Control

LTC3567
APPLICATIONS INFORMATION
V_BUS
R_NOM
100k
NTC
3
V_BUS
LTC3567
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
V_BUS
R_NOM
105k
NTC
3
V_BUS
LTC3567
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
R_NTC
100k
–
0.349 • V_BUS
+
+
0.017 • V_BUS
–
TOO_HOT
NTC_ENABLE
R1
12.7k
R_NTC
100k
–
0.349 • V_BUS
+
+
0.017V • V_BUS
–
TOO_HOT
NTC_ENABLE
3567 F04a
(a)
Figure 4. NTC Circuits
3567 F04b
(b)
it is best to have a low voltage coefficient capacitor at the
V_BUS pin to the LTC3567. This is achievable by selecting an
MLCC capacitor that has a higher voltage rating than that
required for the application. For example, a 16V, X5R, 10μF
capacitor in a 1206 case would be a more conservative
choice than a 6.3V, X5R, 10μF capacitor in a smaller 0805
case. The size of the input overshoot will be determined
by the “Q” of the resonant tank circuit formed by C_IN and
the input lead inductance. It is recommended to measure
the input ringing with the selected components to verify
compliance with the Absolute Maximum specifications.
Alternatively, the following soft connect circuit (Figure 5)
can be employed. In this circuit, capacitor C1 holds MP1
off when the cable is first connected. Eventually C1 begins
to charge up to the USB input voltage applying increasing
gate support to MP1. The long time constant of R1 and
C1 prevent the current from building up in the cable too
fast thus dampening out any resonant overshoot.
5V USB
INPUT _{USB CABLE}
MP1
Si2333
C1
100nF
R1
40k
V_BUS
C2
10μF
LTC3567
GND
3567 F05
Figure 5. USB Soft Connect Circuit
Buck-Boost Regulator Output Voltage Programming
scale output voltage is programmed using a resistor divider
from the V_OUT1 pin connected to the FB1 pin such that:
V_OUT1 =
V_FB1
^
R1
R_FB
+ 1^
where V_FB1 ranges from 0.425V to 0.8V (see Figure 6).
Closing the Feedback Loop
The LTC3567 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
V_IN12
•I_OUT • L
•
V_OUT1
Hz
The buck-boost regulator can be programmed for output
voltages greater than 2.75V and less than 5.5V. The full-
24
The loop gain is typically rolled off before the RHP zero
frequency.
3567f