参数资料
| 型号： | SC824ULTRT |
| 厂商： | Semtech |
| 文件页数： | 21/29页 |
| 文件大小： | 0K |
| 描述： | IC BATTERY MANAGEMENT |
| 标准包装： | 1 |
| 系列： | * |
| 其它名称： | SC824ULDKR |
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SC824
Applications Information (continued)
In dropout, the power dissipation in the SC824 is
P ILIM = (R DS-ON ) x (I AD-LIM ) 2 . Since R DS-ON does not vary with
battery voltage, dropout power dissipation is constant
throughout the CC portion of the charge cycle while the
adapter remains in current limit. The SC824 junction tem-
perature will rise above ambient by P ILIM x θ JA . If the device
temperature rises to the temperature at which the TL
control loop limits charging current (rather than the
current being limited by the adapter), the input voltage
will rise to the adapter regulation voltage. The power dis-
sipation will increase so that the TL regulation will further
limit charge current. This will keep the adapter in voltage
regulation for the remainder of the charge cycle. In this
case, the SC824 will continue to charge with thermal limit-
ing until charge current decreases while in CV regulation
(reducing power dissipation sufficiently), resulting in a
slow charge cycle, but with no other negative effect.
To ensure that the adapter remains in current limit, the
internal device temperature must never rise to T TL . This
implies that θ JA must be kept small enough, through
careful layout, to ensure that T J = T A + (P ILIM × θ JA ) < T TL .
Under-Voltage Load Regulation in USB Modes
The VIN pin UVLR feature, enabled in either USB mode,
prevents the battery charging current from overloading
the USB Vbus network, regardless of the programmed fast
charge value (I FQ_USB ). When either USB High Power or USB
Low Power mode is selected, the SC824 monitors the
input voltage (V VIN ) and reduces the charge current by the
amount necessary to keep V VIN no lower than the UVLR
limit (V UVLR ). UVLR is active only when one of the USB
modes is selected. UVLR ensures the integrity of the USB
Vbus supply for all devices sharing a host or hub supply.
In either USB mode, the UVLR feature will reduce the
charging current to zero if V VIN is externally pulled below
V UVLR . This condition will not be interpreted as termination
and will not result in an end-of-charge indication. The
STAT1 pin will remain asserted as if charging is continuing.
This behavior prevents repetitive indications of end-of-
charge alternating with start-of-charge in the case that
the external VIN load is removed or is intermittent. STAT0
remains asserted until the input voltage is less than
VT UVLO-F .
USB High Power and Low Power Support
The USB specification restricts the load on the USB Vbus
power network to 100mA for low power devices and for
high power devices prior to granting permission for high
power operation. The USB specification restricts the Vbus
load to 500mA for high power devices after granting per-
mission to operate as a high power device. A fixed 1:5
ratio of low power to high power charging current is desir-
able for charging batteries with maximum fast charge
current of at least 500mA. For this application, the SC824
provides fixed 1:5 current ratio low-to-high power mode
support via the tri-level MODE input pin.
For batteries with maximum fast charge current less than
500mA, a fixed 1:5 low/high power charge current ratio
will result in suboptimal charging in USB low power mode.
For example, a 300mAh battery will typically require a
fast-charge current of 300mA or less. A fixed 1:5 ratio for
USB low-to-high power charging current will unnecessar-
ily reduce charging current to 60mA, well below the
100mA permitted. In this case, it may be preferable to
program USB low-power fast-charge current by switching
an external programming resistor. See the section Design
Considerations — Small Battery.
Input Over-Voltage Protection
The VIN pin is protected from over-voltage to at least 30V
above GND. When the input voltage exceeds the Over-
Voltage Protection rising threshold (VT OVP-R ), charging is
halted. Charging resumes when the input voltage falls
below the OVP falling threshold. OVP turns off the STAT0
and STAT1 outputs.
All modes use the same input OVP threshold, which has
been set relatively high to permit the use of poorly regu-
lated adapters. Such adapters may output a high voltage
until loaded by the charger. A too-low OVP threshold
could prevent the charger from ever turning on and
loading the adapter to a lower voltage. If the adapter
voltage remains high despite the charging load, the fast
thermal limiting feature will immediately reduce the
charging current to prevent overheating of the charger.
This behavior is illustrated in Figure 5, in which V BAT = 3.0V,
I FQ = 700mA, and V VIN is stepped from 0V to 8.1V. Initially,
power dissipation in the SC824 is 3.6W.
21
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