LT1505CG-1 Ver la hoja de datos (PDF) - Linear Technology
Número de pieza
componentes Descripción
Lista de partido
LT1505CG-1 Datasheet PDF : 16 Pages
| |||
LT1505
U
OPERATION
The LT1505 is a synchronous current mode PWM step-
down (buck) switcher. The battery DC charge current is pro-
grammed by a resistor RPROG (or a DAC output current) at
the PROG pin and the ratio of sense resistors RS2 over RS1
(see Block Diagram). Amplifier CA1 converts the charge cur-
rent through RS1 to a much lower current IPROG (IPROG =
IBAT • RS1/RS2) fed into the PROG pin. Amplifier CA2 com-
pares 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 supplies the topside power switch gate
drive. The LT1505 generates an 8.6V VGBIAS to power drives
and VBOOSTC. BOOSTC pin supplies the current amplifier
CA1 with a voltage higher than VCC for low dropout appli-
cation. For batteries like lithium that require both constant-
current and constant-voltage charging, 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.
The amplifier CL1 monitors and limits the input current,
normally from the AC adapter, to a preset level (92mV/RS).
At input current limit, CL1 will supply the programming
current IPROG, thus reducing battery charging current.
To prevent current shoot-through between topside and
lowside switches, comparators A3 and A4 assure that one
switch turns off before the other is allowed to turn on.
Comparator A12 monitors charge current level and turns
lowside switch off if it drops below 20% of the programmed
value (20mV across RS1) to allow for inductor discontinu-
ous mode operation. Therefore sometimes even in con-
tinuous mode operation with light current level the lowside
switch stays off.
Comparator E6 monitors the charge current and signals
through the FLAG pin when the charger is in voltage mode
and the charge current level is reduced to 20%. This charge
complete signal can be used to start a timer for charge
termination.
The INFET pin drives an external P-channel FET for low
dropout application.
When input voltage is removed, VCC will be held up by the
body diode of the topside MOSFET. The LT1505 goes into
a low current, 10µA typical, sleep mode as VCC drops
below the battery voltage. To shut down the charger
simply pull the VC pin or SHDN pin low with a transistor.
APPLICATIONS INFORMATION
Input and Output Capacitors
In the 4A Lithium Battery Charger (Figure 1), 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 output charging 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
can be created 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. Highest possible voltage rating on the
capacitor will minimize problems. Consult the manufac-
turer before use. Alternatives include new high capacity
ceramic (at least 20µF) from Tokin or United Chemi-Con/
Marcon, et al.
The output capacitor (COUT) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
( ) IRMS =
0.29
(VBAT)
1
– VBAT
VCC
(L1)(f)
For example, VCC = 19V, VBAT = 12.6V, L1 = 15µH,
and f = 200kHz, IRMS = 0.4A.
10
Share Link: 