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LTC3556

3556f

the LC double pole combined with the –90° of phase lag

from the right-half plane zero will result in negating the

phase bump of the compensator.

The compensator zeros should be placed either before

or only slightly after the LC double pole such that their

positive phase contributions offset the –180° that occurs

at the ﬁlter double pole. If they are placed at too low of a

frequency, they will introduce too much gain to the system

and the crossover frequency will be too high. The two high

frequency poles should be placed such that the system

crosses unity gain during the phase bump introduced

by the zeros and before the boost right-half plane zero

and such that the compensator bandwidth is less than

the bandwidth of the error amp (typically 900kHz). If the

gain of the compensation network is ever greater than

the gain of the error ampliﬁer, then the error ampliﬁer no

longer acts as an ideal op amp, and another pole will be

introduced at the same point.

Recommended Type III compensation components for a

3.3V output:

R1: 324kΩ

RFB: 105kΩ

C1: 10pF

R2: 15kΩ

C2: 330pF

R3: 121kΩ

C3: 33pF

COUT: 22μF

LOUT: 2.2μH

Over-Programming the Battery Charger

The USB high power speciﬁcation allows for up to 2.5W to

be drawn from the USB port (5V

×500mA).ThePowerPath

switching regulator transforms the voltage at VBUS to just

above the voltage at BAT with high efﬁciency, while limiting

power to less than the amount programmed at CLPROG.

In some cases the battery charger may be programmed

(with the PROG pin) to deliver the maximum safe charging

current without regard to the USB speciﬁcations. If there

is insufﬁcient current available to charge the battery at the

programmed rate, the PowerPath regulator will reduce

charge current until the system load on VOUT is satisﬁed

and the VBUS current limit is satisﬁed. Programming the

battery charger for more current than is available will

not cause the average input current limit to be violated.

It will merely allow the battery charger to make use of

all available power to charge the battery as quickly as

possible, and with minimal power dissipation within the

battery charger.

Alternate NTC Thermistors and Biasing

The LTC3556 provides temperature qualiﬁed charging if

a grounded thermistor and a bias resistor are connected

to NTC. By using a bias resistor whose value is equal to

the room temperature resistance of the thermistor (R25)

the upper and lower temperatures are pre-programmed

to approximately 40°C and 0°C, respectively (assuming

a Vishay “Curve 1” thermistor).

The upper and lower temperature thresholds can be ad-

justed by either a modiﬁcation of the bias resistor value

or by adding a second adjustment resistor to the circuit.

If only the bias resistor is adjusted, then either the upper

or the lower threshold can be modiﬁed but not both. The

other trip point will be determined by the characteristics

of the thermistor. Using the bias resistor in addition to an

adjustment resistor, both the upper and the lower tempera-

ture trip points can be independently programmed with

the constraint that the difference between the upper and

lower temperature thresholds cannot decrease. Examples

of each technique follow.

NTC thermistors have temperature characteristics which

are indicated on resistance-temperature conversion tables.

The Vishay-Dale thermistor NTHS0603N011-N1003F, used

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