LTC4088 LINER [Linear Technology], LTC4088 Datasheet - Page 16

• Manufacturer Part Number: LTC4088
• Description: High Effi ciency Battery Charger/USB Power Manager
• Manufacturer: LINER [Linear Technology]

LTC4088
APPLICATIO S I FOR ATIO
CLPROG Resistor and Capacitor
As described in the Step-Down Input Regulator section,
the resistor on the CLPROG pin determines the average
input current limit in each of the six current limit modes.
The input current will be comprised of two components,
the current that is used to drive V
current of the switching regulator. To ensure that the USB
specifi cation is strictly met, both components of input cur-
rent should be considered. The Electrical Characteristics
table gives the typical values for quiescent currents in all
settings as well as current limit programming accuracy.
To get as close to the 500mA or 100mA specifi cations as
possible, a precision resistor should be used.
An averaging capacitor is required in parallel with the
resistor so that the switching regulator can determine
the average input current. This capacitor 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 Inductor
Because the average input current circuit does not measure
reverse current (i.e., current from V
rent reversal in the inductor at light loads will contribute
an error to the V
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 charging and the system load is thus reduced. The
USB specifi cation will not be violated.
This reduction in available V
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, but it does
not affect the average current loop since it will not be in
regulation.
The LTC4088 includes a current-reversal comparator which
monitors inductor current and disables the synchronous
16
BUS
U
current measurement. The error is
U
BUS
current will happen when
OUT
W
OUT
and the quiescent
to V
BUS
U
), cur-
rectifi er 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 reverse slightly. This
value depends on the speed of the comparator in relation
to the slope of the current waveform, given by V
V
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
lighter loads the current-reversal comparator will disable
the synchronous rectifi er at a current slightly above 0mA. As
the inductance is reduced from this value, the part will enter
discontinuous conduction mode at progressively higher
loads. Ripple at V
to the magnitude of inductor ripple. Transient response,
however, will be improved. The current mode controller
controls inductor current to exactly the amount required
by the load to keep V
step requires the inductor current to change to a new level.
Since inductor current cannot change instantaneously, the
capacitance on V
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 (or more C
and a physically larger inductor package size.
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 with reasonable inductor ripple. It will prevent
inductor current runaway in case of a shorted output.
The DC winding resistance and AC core losses of the
inductor will affect effi ciency, and therefore available
output power. These effects are diffi cult to characterize
and vary by application. Some inductors which may be
suitable for this application are listed in Table 3.
L
is the voltage across the inductor (approximately –V
OUT
OUT
delivers or absorbs the difference in
will increase, directly proportionally
OUT
in regulation. A transient load
BUS
VOUT
current. At
L
/L, where
required)
OUT
4088f
)