SC820 datasheet(19/21 Pages) SEMTECH

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SC820

high power devices prior to granting permission for high

power operation. The specification restricts the Vbus load

to 500mA for high power devices after granting permis-

sion to operate as a high power device. This suggests that

a fixed 1:5 ratio of low power to high power charging

current is desirable. But this can result in suboptimal

charging when the battery capacity is too small to permit

fast charging at 500mA. For example, a 250mAh battery

will typically require a fast-charge current of 250mA or

less. A fixed 1:5 ratio for USB low and high power charging

will unnecessarily reduce charging current to 50mA, well

below the 100mA permitted.

An arbitrary ratio of USB low-to-high power charging cur-

rents can be obtained using an external n-channel FET

operated with a processor GPIO signal to engage a second

parallel IPUSB resistor. The external circuit is illustrated in

Figure 5.

IPUSB

RIPUSB

RIPUSB_HI

USB Hi/Lo

Power Select

Figure 5 — External programming of arbitrary USB

high power and low power charge currents.

For USB low power mode charging, the external transistor

is turned off. The transistor is turned on when high power

mode is desired. The effect of the switched parallel IPUSB

resistor is to reduce the effective programming resistance

and thus raise the fast-charge current.

An open-drain GPIO can be used directly to engage the

parallel resistor R

IPUSB_HI

. Care must be taken to ensure that

the R

DS-ON

of the GPIO is considered in the selection of

IPUSB_HI

. Also important is the part-to-part and tempera-

ture variation of the GPIO R

DS-ON

, and their contribution to

the USB High Power charge current tolerance. Note also

that IPUSB will be pulled up briefly to as high as 3V during

startup to check for an IPUSB static pinshort to ground. A

small amount of current could, potentially, flow from

IPUSB into the GPIO ESD structure through R

IPUSB_HI

during

this event. While unlikely to do any harm, this effect must

also be considered.

For purposes of design for dual-input adapter/USB charg-

ing, a small battery is one with a desired fast-charge

current less than 500mA. A 300mAh battery with

maximum fast-charge current of 300mA is an example.

The adapter input and USB input high power fast-charge

currents should both be set to 300mA max. The USB input

low power fast-charge current is 100mA max. Refer to the

circuit of Figure 4 and the data of Figures 1a and 1b. For

FQ_AD

= 300mA max, use R

IPRGM

= 7.50kΩ. The fixed IPUSB

resistor of R

IPUSB

= 23.2kΩ programs I

FQ_USB

= 100mA max.

When parallel resistor R

IPUSB_HI

= 11.0kΩ is switched in, the

equivalent IPUSB resistor is 7.50kΩ, and so I

FQ_USB

= 300mA

max.

A large battery is any battery with a desired fast-charge

current exceeding 500mA. Large battery charging is most

consistent with the USB fixed 1:5 current ratio low-to-high

power model of operation. For example, consider an

800mAh battery, with maximum fast-charge current of

800mA. The adapter input fast-charge should be config-

ured for 800mA max (R

IPRGM

= 2.80kΩ), the USB low power

fast-charge set to 100mA max (R

IPUSB

= 23.2kΩ), and the

USB high power fast- charge set to 500mA max

IPUSB_HI

= 5.62kΩ).

USB Low Power Mode Alternative

Where a USB mode selection signal is not available, or

where system cost or board space make USB low power

mode external current programming impractical, USB low

power charging can be supported indirectly. The IPUSB

pin resistance can be selected to obtain the desired USB

high power charge current. The VUSB pin UVLR feature

ensures that the charging load will never pull the USB

Vbus supply voltage below V

USB_UV_LIM

regardless of the USB

host or hub supply limit. The UVLR limit voltage guaran-

tees that the voltage of the USB Vbus supply will not be

loaded below the low power voltage specification limit, as

seen by any other low power devices connected to the

same USB host or hub.

Applications Information (continued)

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