RT7800B
Constant-Current (CC) Regulation
production, the RT7800B based products can use same-
version RT7800B ICs to reduce inventory cost. It also
allows updating RT7800B firmware at end customer site
through internet in response to some necessary system
software changes.
In a RT7800B inside Provider working in coordination with
aDC-DC converter, the output CC regulation function can
be implemented by firmware design. The RT7800B,
continuously monitoring output current via CSP and CSN
pins, can regulate output current at a selected current
level by dynamically adjusting the digitalized current
flowing into DAC_CV pin. The CC regulation, limiting
VBUS output current, provides a better system protection
than over-current protection.
Power Output for USB Plug Power (VCONN
)
In Figure 3, a selected output voltage at one of VCONN1
and VCONN2 pins is provided for powering an external
Electrically Marked (E-mark) or active cable. One of
internal MOSFETs between VCONN to VCONN1 and
VCONN2 pins can be turned on to supply power to
VCONN1 or VCONN2 pin. Connect the VCONN1/2 pin to
USB-C CC1/2 terminal via a Schottky diode (D1/2) for
blocking reverse current. The input pin VCONN must be
connected to a 5V power source.
Power-Path Gate Drivers
There are four gate drivers in the RT7800B to turn on/off
three pairs of low on-resistance N-Channel power
MOSFETs for each individual input/output power path.
Compared to driving P-Channel power MOSFET, this design
usingN-MOSFET is more cost-effective. Each gate driver
includes a built-in charge pump for turning on and an internal
pull-low switch for turning off the power MOSFETs.
Dead Battery Function
In Figure 3, a voltage-clamping circuit clamps the CC1/2
voltage when the RT7800B has no power at VBUS, VX
and VSYS5 pins. A no-power RT7800B Provider or Dual
Role port will be recognized as a Consumer by clamping
the voltage level at the RT7800B CC1/2 pin. The RT7800B
starts to work, after an external Provider supplies USB
cable power (VBUS) to its VBUS pin.
Internet On-line Firmware Update
Due to using MTP memory, RT7800B firmware can be
updated by an EC (Embedded Controller) or AP
(Application Processor) via I2C slave interface. Users can
easily update firmware without de-soldering/soldering
RT7800B during product development period. In mass
5V
VCONN
ON1/2
VCONN1/2
Selectable
Current-Source (Ip)
(off, 80µA, 180µA or
330µA)
Switch
VCONN1/2
D1/2
SEL1 to 2
MCU
System
CC1/2
To USB-C
CC1/2 terminal
ADC
SW3
Transmitter
Receiver
Rd
5.1k
Voltage
Clamping
at no bias input
BMC
Encoder/
Decoder
Dead-Battery Function
BMC : Biphase Mark Coding
PD Communication Physical Layer
Figure 3
Copyright 2017 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
8
DS7800B-00 May 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
