參數(shù)資料
型號(hào)： LTC4012IUF-1#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 633 kHz SWITCHING FREQ-MAX, PQCC20
封裝： 4 X 4 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-220WGGD-1, QFN-20
文件頁(yè)數(shù)： 11/28頁(yè)
文件大小： 381K
代理商： LTC4012IUF-1#TRPBF
4012fa
LTC4012/
LTC4012-1/LTC4012-2
applications inForMation
The first option is to lower the power dissipation of RCL at
theexpenseofaccuracywithoutchangingtheinputcurrent
limit value. The second is to make the input current limit
value programmable.
The overall accuracy of this circuit needs to be better
than the power source current tolerance or be margined
such that the worse-case error remains under the power
source limits.
The accuracy of the Figure 7 circuit is a function of the
INTVDD, VBE, RCL, RF, R1 and R3 tolerances. To improve
accuracy, the tolerance of RF should be changed from
5.1k, 5% to a 2.49k 1% resistor. RCL and the programming
resistors R1 and R3 should also be 1% tolerance such
that the dominant error is INTVDD (±3%). Bias resistor R2
can be 5%. When choosing NPN transistors, both need
to have good gain (>100) at 10A levels. Low gain NPNs
will increase programming errors. Q1 must be a matched
NPN pair. Since RF has been reduced in value by half, the
capacitor value of CF should double to 0.22F to remain
effective at filtering out any noise.
If you wish to reduce RCL power dissipation for a given
current limit, the programming equation becomes:
R
mV
k
R
I
CL
LIM
=
100
5 2 49
1
–
.
If you wish to make the input current limit programmable,
the equation becomes:
I
mV
k
R
LIM
CL
=
100
5 2 49
1
–
.
The equation governing R2 for both applications is based
on the value of R1. R3 should always be equal to R1.
R2 = 0.875 R1
Figure 8. PROG Voltage Buffer
17
13
INTVDD
PROG
<30nA
LTC4012
4012 F08
TO SYSTEM
MONITOR
+
–
In many notebook applications, there are situations
where two different ILIM values are needed to allow two
different power adapters or power sources to be used.
In such cases, start by setting RLIM for the high power
ILIM configuration and then use Figure 7 to set the lower
ILIM value. To toggle between the two ILIM values, take
the three ground connections shown in Figure 7, combine
them into one common connection and use a small-signal
NFET (2N7002) to open or close that common connec-
tion to circuit ground. When the NFET is off, the circuit
is defeated (floating) allowing ILIM to be the maximum
value. When the NFET is on, the circuit will become active
and ILIM will drop to the lower set value.
Monitoring Charge Current
The PROG pin voltage can be used to indicate charge cur-
rentwhere1.2085Vindicatesfullprogrammedcurrent(1C)
and zero charge current is approximately equal to RPROG
11.67A. PROG voltage varies in direct proportion to the
chargecurrentbetweenthiszero-current(offset)valueand
1.2085V. When monitoring the PROG pin voltage, using a
buffer amplifier as shown in Figure 8 will minimize charge
current errors. The buffer amplifier may be powered from
the INTVDD pin or any supply that is always on when the
charger is on.
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