Datasheet

LTC3556
25
3556f
Buck-Boost Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
reference voltage input to the error amplifi er over a 0.5ms
(typical) period. This limits transient inrush currents during
start-up because the output voltage is always “in regula-
tion.” Ramping the reference voltage input also limits the
rate of increase in the V
C3
voltage which helps minimize
output overshoot during start-up. A soft-start cycle oc-
curs whenever the buck-boost is enabled, or after a fault
condition has occurred (thermal shutdown or UVLO). A
soft-start cycle is not triggered by changing operating
modes. This allows seamless operation when transitioning
between Burst Mode operation and PWM mode.
Low Supply Operation
The LTC3556 incorporates an undervoltage lockout circuit
on V
OUT
(connected to V
IN3
) which shuts down the buck-
boost regulator when V
OUT
drops below 2.6V. This UVLO
prevents unstable operation.
OPERATION
APPLICATIONS INFORMATION
CLPROG Resistor and Capacitor
As described in the High Efficiency Switching PowerPath
Controller section, the resistor on the CLPROG pin deter-
mines the average input current limit when the switching
regulator is set to either the 1x mode (USB 100mA), the
5x mode (USB 500mA) or the 10x mode. The input cur-
rent will be comprised of two components, the current
that is used to drive V
OUT
and the quiescent current of the
switching regulator. To ensure that the USB specification
is strictly met, both components of input current should
be considered. The Electrical Characteristics table gives
values for quiescent currents in either setting as well as
current limit programming accuracy. To get as close to
the 500mA or 100mA specifications as possible, a 1%
resistor should be used. Recall that I
VBUS
= I
VBUSQ
+
V
CLPROG
/R
CLPPROG
• (h
CLPROG
+1).
An averaging capacitor or an R-C combination is required
in parallel with the CLPROG resistor so that the switching
regulator can determine the average input current. This
network also provides the dominant pole for the feedback
loop when current limit is reached. To ensure stability,
the capacitor on CLPROG should be 0.47µF or larger.
Alternatively, faster transient response may be achieved
with 0.1µF in series with 8.2.
Choosing the PowerPath Inductor
Because the input voltage range and output voltage range
of the power path switching regulator are both fairly nar-
row, the LTC3556 was designed for a specifi c inductance
value of 3.3µH. Some inductors which may be suitable
for this application are listed in Table 6.
Table 6. Recommended Inductors for PowerPath Controller
INDUCTOR
TYPE
L
(μH)
MAX
I
DC
(A)
MAX
DCR
(Ω)
SIZE IN mm
(L × W × H) MANUFACTURER
LPS4018 3.3 2.2 0.08
3.9 × 3.9 × 1.7
Coilcraft
www.coilcraft.com
D53LC
DB318C
3.3
3.3
2.26
1.55
0.034
0.070
5.0 × 5.0 × 3.0
3.8 × 3.8 × 1.8
Toko
www.toko.com
WE-TPC
Type M1
3.3 1.95 0.065
4.8 × 4.8 × 1.8
Würth Elektronik
www.we-online.com
CDRH6D12
CDRH6D38
3.3
3.3
2.2
3.5
0.0625
0.020
6.7 × 6.7 × 1.5
7.0 × 7.0 × 4.0
Sumida
www.sumida.com
V
BUS
and V
OUT
Bypass Capacitors
The style and value of capacitors used with the LTC3556
determine several important parameters such as regulator
control-loop stability and input voltage ripple. Because
the LTC3556 uses a step-down switching power supply
from V
BUS
to V
OUT
, its input current waveform contains
high frequency components. It is strongly recommended
that a low equivalent series resistance (ESR) multilayer
ceramic capacitor be used to bypass V
BUS
. Tantalum and
aluminum capacitors are not recommended because of
their high ESR. The value of the capacitor on V
BUS
directly
controls the amount of input ripple for a given load cur-
rent. Increasing the size of this capacitor will reduce the
input ripple.