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LT1571 Series

The LT1571 is a current mode PWM step-down (buck)

charger. The battery charge current is programmed by a

resistor RPROG (or a DAC output current) at the PROG pin

(see Block Diagram). Amplifier CA1 converts the charge

current through RS1to a much lower current IPROG(500µA/

A) fed into the PROG pin. Amplifier CA2 compares the

output of CA1 with the programmed current and drives the

PWM loop to force them to be equal. High DC accuracy is

achieved with averaging capacitor CPROG. Note that IPROG

has both AC and DC components. IPROG goes through R1

and generates a ramp signal that is fed to the PWM control

comparator C1 through buffer B1 and level shift resistors

R2 and R3, forming the current mode inner loop. The

BOOST pin drives the NPN switch (QSW) into saturation

and reduces power loss. For batteries like lithium-ion that

require both constant-current and constant-voltage charg-

ing, the 0.5%, 2.465V reference and the amplifier VA

reduce the charge current when battery voltage reaches

the preset level. For NiMH and NiCd, VA can be used for

overvoltage protection. When input voltage is removed,

the VCC pin drops to 0.7V below the battery voltage forcing

the charger into a low-battery drain (5

µA typical) sleep

mode. To shut down the charger, simply pull the VC pin low

with a transistor.

Comparator E6 monitors the charge level and signals

through the FLAG pin when charging is in voltage mode

and the charge current has reduced to 20% or less. This

charge complete signal can be used to start a timer for

charging termination.

Input and Output Capacitors

In the charger circuits in Figures 1 and 2, the input

capacitor CIN is assumed to absorb all input switching

ripple current in the converter, so it must have adequate

ripple current rating. Worst-case RMS ripple current will

be equal to one half of the output charge current. Actual

capacitance value is not critical. Solid tantalum capacitors

such as the AVX TPS and Sprague 593D series have high

ripple current rating in a relatively small surface mount

package, but

caution must be used when tantalum capaci-

tors are used for input bypass. High input surge currents

are possible when the adapter is hot-plugged to the

charger and solid tantalum capacitors have a known

failure mechanism when subjected to very high turn-on

surge currents. Selecting a high voltage rating on the

capacitor will minimize problems. Consult with the manufac-

turer before use. Alternatives include new high capacity

ceramic capacitors from Tokin or United Chemi-Con/

MARCON, et al. OS-CON can also be used.

The output capacitor COUT is also assumed to absorb

output switching ripple current. The general formula for

capacitor ripple current is:

RMS

BAT

() −









()( )

029

For example, with VCC = 16V, VBAT = 8.4V, L1 = 33µH and

f = 200kHz, IRMS = 0.18A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads. Beads or inductors can

be added to increase battery impedance at the 200kHz

switching frequency. Switching ripple current splits

between the battery and the output capacitor depending

on the ESR of the output capacitor and the battery imped-

ance. If the ESR of COUT is 0.2Ω and the battery impedance

is raised to 4

Ω with a bead of inductor, only 5% of the

ripple current will flow into the battery.

Soft-Start

The LT1571 is soft-started by the 0.3

µF capacitor on VC

pin. On start-up, the VC pin voltage will rise quickly to 0.5V,

then ramp at a rate set by the internal 45

µA pull-up current

and the external capacitor. Charge current starts ramping

up when the VC pin voltage reaches 0.9V and full current

OPERATIO

APPLICATIO S I FOR ATIO

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