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参数资料
| 型号： | LT3651IUHE-4.1#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | BATTERY CHARGE CONTROLLER, 1100 kHz SWITCHING FREQ-MAX, PQCC36 |
| 封装： | 5 X 6 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, QFN-36 |
| 文件页数： | 4/22页 |
| 文件大小： | 213K |
| 代理商： | LT3651IUHE-4.1#PBF |
LT3651-4.1/LT3651-4.2
12
365142fb
APPLICATIONS INFORMATION
OSC Frequency
A precision resistor to ground sets the LT3651 switcher
oscillator frequency, fOSC, permitting user adjustability
of the frequency value. Typically this frequency is in the
200kHz to 1MHz range. Power consideration may neces-
sitate lower frequency operation especially if the charger
is operated with very high voltages. Adjustability also
allows the user to position switching harmonics if their
system requires.
The timing resistor, RT, value is set by the following:
RT =
54.9
fOSC MHz
()
k
Ω
()t
Set RT to 54.9k for 1MHz operation.
VIN Input Supply
The LT3651 is biased directly from the charger input supply
through the VIN pin. This supply provides large switched
currents, so a high quality, low ESR decoupling capacitor
is required to minimize voltage glitches on VIN. The VIN
decoupling capacitor (CVIN) absorbs all input switching
ripple current in the charger. Size is determined by input
ripple voltage with the following equation:
CIN(BULK) =
IMAX VBAT
fOSC MHz
() ΔV
IN VIN
μF
( )
where ΔVIN is the input ripple, IMAX is the maximum
charge current and f is the oscillator frequency. A good
starting point for ΔVIN is 0.1V. Worst-case conditions are
with VBAT high and VIN at minimum. So for a 8V VIN(MIN),
IMAX = 4A and a 1MHz oscillator frequency:
CIN(BULK) =
4 4.2
1 0.1 8
= 21μF
The capacitor must have an adequate ripple current rating.
RMS ripple current, ICVIN(RMS) is approximated by:
ICVIN(RMS) ≈ ICHARGE(MAX)
VBAT
VIN
VIN
VBAT
–1
which has a maximum at VIN = 2 VBAT, where ICVIN(RMS)
= ICHARGE(MAX)/2. In the example above that requires a
capacitor RMS rating of 2A.
Boost Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the high side switch
transistor. The BOOST voltage is normally created by con-
necting a 1μF capacitor from the BOOST pin to the SW
pin. Operating range of the BOOST pin is 2V to 4.5V, as
referenced to the SW pin.
The boost capacitor is normally charged via a diode con-
nected from the battery or an external source through the
low side switch. Rate the diode average current greater
than 0.1A and its reverse voltages greater than VIN(MAX).
If an external supply that is greater than the input is avail-
able (VBOOST – VIN > 2V), it may be used in place of the
bootstrap capacitor and diode.
VIN ,VBOOST Start-Up Requirement
The LT3651 operates with a VIN range of 4.8V to 32V. The
charger begins a charging cycle when the detected battery
voltage is below the 4.0V/4.1V auto-restart float voltage
and the part is enabled.
When VIN is below 6.3V and the BOOST capacitor is un-
charged, the high side switch would normally not have
sufficient head room to start switching. During normal
operation the low side switch is deactivated when charge
current is very low to prevent reverse current in the in-
ductor. However in order to facilitate start-up, the LT3651
enables the switch if VBOOST voltage is low. This allows
initial charging of the BOOST capacitor which then permits
the high side switch to saturate and efficiently operate. The
boost capacitor charges to full potential after a few cycles.
The design should consider that as the switcher turns on
and input current increases, input voltage drops due to
source input impedance and input capacitance. This po-
tentially allows the input voltage to drop below the internal
VIN UVLO turn-on and thus disrupt normal behavior and
potentially stall start-up. If an input current sense resis-
tor is used, its drop must be considered as well. These
problems are worsened because input current is largest
at low input voltage. Pay careful attention to drops in the
power path. Adding a soft-start capacitor to the RNG/SS
pin and setting UVLO to 4.8V with the SHDN pin is required
at low VIN.
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