2011 - 2014 Microchip Technology Inc. DS00001824A-page 13
USB375x
4.1 VBUS Protection Switch
The USB375x protects the entire system from errant VBUS voltages, whether over-voltage or under-voltage conditions.
The device is able to dynamically monitor the VBUS voltage levels and take appropriate action by opening the integrated
VBUS Protection Switch when these errant conditions occur. Specifically, when VBUS is below the Under Voltage Lock-
Out (UVLO) level or above the Over Voltage Lock-Out (OVLO) level, the VBUS switch will open thereby disconnecting
the VBUS pin from VOUT. See Section 3.3 for UVLO and OVLO timing diagrams. Manual control of the VBUS Switch,
as well as switch configuration can be handled through the I
2
C interface. See Section 4.4.2 for I
2
C register descriptions.
4.1.1 100MA CURRENT LIMIT (USB375X-1 ONLY)
bit The USB375x-1 provides a 100mA current limit feature which allows a portable system with a dead battery to still
draw 100mA from a compliant host. When plugged into a Standard Downstream Port (SDP), the USB375x-1 will enable
the 100mA current limit until the system controller enables the OVP switch in the I
2
C registers of the USB375x-1. When
plugged into any other charger, the USB375x-1 will enable the 100mA current limit during charger detection, and will
disable the 100mA current limit once charger detection has completed. Figure 4-6 details the charger detection flow of
the USB375x-1.
4.2 Charger Detection
The USB375x includes the circuitry required to implement the USF-IF Battery Charging Specification (revision 1.2). The
USB375x will automatically perform a charger detection upon start-up. The USB375x includes a state machine to pro-
vide the detection of a wide variety of USB charger detection methods listed in Table 4-1. When any of the USB chargers
are plugged into the system, the USB375x will detect a charger plug in event, determine what type of charger it is, and
write the type of charger information into the I
2
C register as shown in the following table. In addition, there are two meth-
ods to alert the system that a charger has been detected by the USB375x. The first was through the I
2
C register as
mentioned above. The second is through the INT_B pin. When a compliant charger is plugged in and detected, the
USB375x will automatically drive the INT_B signal low. Figure 4-4 details the automatic charger detection flow of the
USB3750-1, Figure 4-5 details the automatic charger detection flow of the USB3751-1, and Figure 4-6 details the auto-
matic charger detection flow of the USB3751-2.
FIGURE 4-2: USB3751 BLOCK DIAGRAM
SCL
SDA
VBUS
VOUT
I
2
C
Interface
INT_B
DP
DM
ESD Protection
USB Connector
USB 2.0 PHY,
Processor, or
Accessory
Processor
Charger
Detection
Battery
Charger/
PMU
GND
DM_1
DP_1
Over-Voltage/Under-Voltage
Protection and Current
Limiting
VBUS
VBUS
DP
DM
USB3751
