SC824 datasheet(21/29 Pages) SEMTECH

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SC824

In dropout, the power dissipation in the SC824 is

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

. 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

. This

implies that θ

must be kept small enough, through

careful layout, to ensure that T

= T

+ (P

ILIM

× θ

) < T

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

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

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,

= 700mA, and V

VIN

is stepped from 0V to 8.1V. Initially,

power dissipation in the SC824 is 3.6W.

Applications Information (continued)

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