LTC3555/LTC3555-X
25
3555fe
For more information www.linear.com/LTC3555
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 8.2Ω.
Choosing the PowerPath Inductor
Because the average input current circuit does not measure
reverse current (i.e., current from SW to V
BUS
), current
reversal in the inductor at light loads will contribute an
error to the average V
BUS
current measurement. The error
is conservative in that if the current reverses, the voltage
at CLPROG will be higher than what would represent the
actual average input current drawn. The current available
for battery charging plus system load is thus reduced but
the USB specification will not be violated.
This reduction in available V
BUS
current will happen when
the peak-peak inductor ripple is greater than twice the
average current limit setting. For example, if the average
current limit is set to 100mA, the peak-peak ripple should
not exceed 200mA. If the input current is less than 100mA,
the measurement accuracy may be reduced. However, this
will not affect the average current loop since it will not be
in regulation.
The LTC3555 family includes a current-reversal com-
parator which monitors inductor current and disables the
synchronous rectifier as current approaches zero. This
comparator will minimize the effect of current reversal
on the average input current measurement. For some low
inductance values, however, the inductor current may still
reverse slightly. This value depends on the speed of the
comparator in relation to the slope of the current wave-
form, given by V
L
/L. V
L
is the voltage across the inductor
(approximately –V
OUT
) and L is the inductance value.
An inductance value of 3.3μH is a good starting value. The
ripple will be small enough for the regulator to remain in
continuous conduction at 100mA average V
BUS
current.
At lighter loads the current-reversal comparator will dis-
able the synchronous rectifier for currents slightly above
0mA. As the inductance is reduced from this value, the
LTC3555 family will enter discontinuous conduction mode
at progressively higher loads. Ripple at V
OUT
will increase
directly proportionally to the magnitude of inductor ripple.
Transient response, however, will improve. The current
mode controller controls inductor current to exactly the
amount required by the load to keep V
OUT
in regulation. A
transient load step requires the inductor current to change
to a new level. Since inductor current cannot change instan-
taneously, the capacitance on V
OUT
delivers or absorbs the
difference in current until the inductor current can change
to meet the new load demand. A smaller inductor changes
its current more quickly for a given voltage drive than a
larger inductor, resulting in faster transient response. A
larger inductor will reduce output ripple and current ripple,
but at the expense of reduced transient performance and
a physically larger inductor package size. For this reason
a larger C
VOUT
will be required for larger inductor sizes.
The input regulator has an instantaneous peak current
clamp to prevent the inductor from saturating during tran-
sient load or start-up conditions. The clamp is designed
so that it does not interfere with normal operation at high
loads and reasonable inductor ripple. It is intended to pre-
vent inductor current runaway in case of a shorted output.
The DC winding resistance and AC core losses of the in-
ductor will affect efficiency, and therefore available output
power. These effects are difficult to characterize and vary
