NEW PRODUCTS 5
Pch HIGH-SIDE SWITCH FOR USB
µPD16855
HOST
(PC)
Takahiro Hattori
HUB (PC Monitor)
Scanner
USB HUB
Printer
Modem
CCD Camera
Keyboard
Mouse
PC peripheral units can be connected with a standardized cable.
USB line
Fig. 1 USB System
Introduction
Personal computers (PCs) have become
widespread in recent years, and new
proposals have been made concerning the
interfaces that connect a PC and peripheral
units. These proposals include standards
such as the Universal Serial BUS (USB) and
IEEE1394 intended to supersede the existing
interfaces. Figure 1 shows a USB system.
NEC has developed many devices that
conform to the USB and IEEE1394
standards, such as the
µPD72011
USB hub
controller IC and the
µPD789800
keyboard
microcontroller with USB capability.
When designing a USB-supporting unit,
a power management circuit is essential for
obtaining power from the USB port and
supplying it to the peripheral devices.
The
µPD16855
Pch high-side switch for
USB developed this time is an intelligent
power IC that supplies power to peripheral
devices, which is a key requirement for USB-
supporting units, and overcurrent limiting
operation in case of an abnormality. By
using this IC with a USB controller IC,
system designers can easily design USB-
supporting units.
Why Is Pch High-Side Switch for USB
Necessary?
A high-side switch having an overcurrent
limiting function for power management is
necessary for a USB supporting unit. This
is because the USB standard stipulates the
following concerning the power supply:
• HOST and Self-Powered HUB
Published July 1998
General-Purpose Device Engineering Department, Semiconductor Solution Engineering Division
IN
(input)
7
OUT1
8
(output 1)
5
OUT2
(output 2)
Reference
Voltage
Overcurrent
Detection
Block
2
FLG1
(flag output 1)
Reference
Voltage
Gate Control
Gate Control
Overcurrent
Detection
Block
Under Voltage
Locked Out
Circuit
3
FLG2
(flag output 2)
Thermal Shutdown
Circuit
6
1
4
GND
CTL1
(control input 1)
CTL2
(control input 2)
Fig. 2 Internal Block Diagram of
µPD16855
Supply voltage: 4.75 to 5.25 V
Supply current: 0.5 A MAX (per port)
5 A MAX (per unit)
• Bus-Powered HUB
Supply voltage: 4.40 to 5.25 V
Supply current: 0.1 A MAX (per port)
0.5 A MAX (per unit)
Because of this standard, a USB-
supporting unit is required to have a 5-V
power supply circuit, current detection
circuit, and current limiting circuit.
The
µPD16855
Pch high-side switch for
USB developed this time is an intelligent
power IC that implements the above
functions with a single chip. This IC helps
you design a power circuit conforming to the
USB standard.
Product Outline
The
µPD16855
is a Pch high-side switch
for USB conforming to the above USB
standard. It operates at 5 V and contains two
switch circuits.
Because it is provided with an overcurrent
detection and current limiting circuits
corresponding to each switch, it can
independently control a USB port. The result
of detection can be reported to a USB
controller by the signal output from a
detection flag pin. In turn, control signals
are input from the USB controller to each
switch to turn ON/OFF the switch.
The
µPD16855
also has a thermal
shutdown circuit and an under voltage locked
out circuit to prevent the system from
malfunctioning due to malfunctions of
the IC.
The features of the
µPD16855
are as
follows:
• Two circuits of Pch power MOS FETs
• Overcurrent detection circuit outputting
signal from detection flag pin
• Overcurrent limiting circuit to prevent
voltage drop in system due to overcurrent
• Thermal shutdown circuit
• Under voltage locked out circuit
• Soft start function
• Switches turned ON/OFF by control pin
• Space-saving 8-pin SOP package
Figure 2 shows the block diagram of the
µPD16855,
and Photograph 1 shows its
appearance.
Photo 1 Appearance of
µPD16855
Features
The features of the
µPD16855
Pch high-
side switch for USB are described in detail
below.
1. Two circuits of Pch power MOS FETs
The
µPD16855
has two circuits of internal
Pch power MOS FETs. These Pch
power MOS FETs have an ON resistance of
100 mΩ TYP and 130 mΩ TYP.,
respectively.
According to the USB standard, the
voltage drop due to switching must be
70 mV or less. Therefore, the permissible
ON resistance specification for a switch
when a maximum current of 0.5 A flows is
140 mΩ (= 0.07 (V)/0.5 (A) x 1,000).
The reason for providing a two-switch
circuit is that PCs and PC peripheral units
supporting USB mainly support two ports.
Therefore, only one
µPD16855
is used in a
unit that supports two USB connector ports.
2. Overcurrent detection circuit out-
putting signal from detection flag pin
The
µPD16855
has an overcurrent
detection circuit for each switch.
The overcurrent detection circuit detects
a current value through comparison between
a reference voltage created from an input
voltage with an output voltage, and judges
that an overcurrent has occurred if the
detected current value exceeds a certain
limit.
The accuracy of this overcurrent detection
is 0.6 to 1.25 A, which is slightly higher than
the 0.5 A defined by the USB standard. This
is because, in the case of a USB unit
requiring a current exceeding 0.5 A, too high
a sensitivity in detecting overcurrents is
inconvenient for PC users. By setting the
overcurrent detection value to 0.6 A, slightly
higher than the USB standard value of
0.5 A, the user can use the IC more easily.
The moment a USB unit is connected, a
current called inrush current, which is very
high (up to 3 A) flows, but only momentarily
(up to 10
µs).
If this inrush current is
detected as an overcurrent, the USB is no
longer capable of “plug-and-play
connection”, which is its biggest selling
point. The
µPD16855,
therefore, has a delay
of 10
µs
TYP. for detection time so as not to
detect the inrush current as an overcurrent.
The overcurrent detection result is output
to the Flg pin by an open-drain Nch MOS
FET. Usually, this pin is kept “H” by an
external pull-up resistor, and goes “L” upon
detection of an overcurrent. The detection
result is fed to the USB controller IC to
inform the USB system of the occurrence of
an overcurrent. Figure 3 shows the
overcurrent detection operation.
3. Overcurrent limiting circuit to prevent
voltage drop in system due to
overcurrent
The switches of the
µPD16855
remains
ON even after the IC has detected an
overcurrent and reported the detection result,
unless control by a control pin to be
described later is performed, or a protection
circuit is activated. In this case, the
µPD16855
limits the current by switching
the ON resistance of a switch. By switching
the ON resistance, the output voltage drops
but the current can be limited. This function
prevents the supply voltage of the system
from dropping (= system down), which is
caused by a high current flowing through the
system.
4. Thermal shutdown circuit
The power consumption of the
µPD16855
due to its heat generation can be calculated
as indicated below, using the following
values :
• Maximum power consumption of IC:
Approx. 70 mW (internal circuitry:
approx. 5 mW + two switch circuits:
65 mW MAX.)
• Thermal resistance of 8-pin SOP: Approx.
240°C/W (measured value for reference
only)
Therefore, the temperature rises
approximately 17°C (= 0.07 (W) x 240
(°C/W)). The maximum ambient
temperature is rated at +85°C, which is much
less than the rated chip channel temperature
(150°C).
However, there is a possibility that the
operating temperature will rise beyond the
rated channel temperature due to internal
heat generation in case the ambient
temperature exceeds the rated value by
accident. To provide for this situation, the
µPD16855
has an internal thermal shutdown
circuit. This circuit shuts off the internal
circuitry to protect the IC from damage if
the internal channel temperature exceeds
150°C. The thermal shutdown circuit
monitors the internal channel temperature
(Tch). If Tch exceeds 150°C, the output pins
are turned off and the detection plug pin goes
low, regardless of the input of the control
pins. Figure 4 shows the thermal shutdown
circuit operation.
5. Under voltage locked out circuit
The
µPD16855
is assumed to operate at
5 V. Its recommended operating voltage
range is 4 to 5.5 V.
Usually, the output status of an IC
IN (input)
5V
GND
5V
GND
5V
GND
5V
OUT
(output)
Flg
(output)
CTL
(input)
GND
lout
Overcurrent Detection Threshold
Overcurrent Detected
Fig. 3 Overcurrent Detection Operation
IN (input)
5V
GND
5V
GND
5V
GND
5V
GND
Tch
Operating Temperature (rising) of Thermal
Shutdown Circuit
Operating Temperature (falling) of Thermal
Shutdown Circuit
Standby Status
Non-Standby Status
OUT
(output)
Flg
(output)
CTL
(input)
Fig. 4 Thermal Shutdown Circuit Operation
5V
IN (input)
OUT
(output)
Flg
(output)
GND
5V
GND
5V
GND
5V
GND
Overcurrent Detection
Threshold
CTL
(input)
lout
Fig. 5 Operation at Power ON/OFF
becomes unstable and the IC malfunctions
if the supply voltage drops. Therefore, the
µPD16855
has an under voltage locked out
circuit that stops the IC if the supply voltage
drops below a certain level (3.5 V TYP.).
This circuit prevents the
µPD16855
from
malfunctioning even if an abnormality such
as instantaneous power failure occurs. While
the under voltage locked out circuit operates,
the output is turned off and the detection flag
pin goes “L”.
6. Soft start function
In a USB system, a capacitor of about
150
µF
is connected to the power output pin
of the USB connector. On power application,
a high current (of up to 3 A) flows to charge
this capacitor, and this current may be
detected as an overcurrent by mistake.
Therefore, the
µPD16855
performs soft start
control of the output voltage immediately
after power application (output rise time: 2.5
ms MIN.), so that a high current does not
flow through the capacitor connected to the
output pin and an overcurrent is not detected
by mistake. In the condition where the
output of this product is turned ON, the
output voltage is raised slowly (2.5 ms
MIN.). In the condition where the output is
turned OFF, the voltage of the output
abruptly drops. Figure 5 shows the operation
when the power is turned ON/OFF.
7. Switches turned ON/OFF by control
pin
As described above, the
µPD16855
does
not turn off the switch by itself even if it
detects an overcurrent. This is because, if
the power is arbitrarily turned off, power
cannot be effectively managed from the
viewpoint of the system.
Therefore, the
µPD16855
has a control pin
that is used to turn off the switch. Power
supply to the IC can be controlled by
inputting a signal to this pin from a
controller. When both switch circuits of the
µPD16855
are turned off by the control
signal, the IC enters the standby status. At
this time, the current consumption drops to
5
µA
MAX. When the switches are turned
on again by the control signal, the soft start
function described above is activated.
8. Space-saving 8-pin SOP package
The
µPD16855
is housed in an 18-pin SOP
(225 mils) package that is useful for saving
the space in an application set.
Application Examples
Having the above features, the
µPD16855
can be used for applications using a USB
system.
Figure 6 to 8 show examples of
applications and circuits of various USB
systems.
Figure 6 shows an application example of
the
µPD16855
used in a PC (especially,
notebook type PC). Figure 7 shows an
example used in a PC monitor. Figure 8
shows an application example in a USB hub.
Future Development
USB-supporting systems are expected to
rapidly become widespread after Windows
98 is released. The
µPD16855
is an
intelligent power IC that is ideal for
configuring a USB power system
conforming to the USB standard. NEC has
developed this USB power IC to satisfy
market demands. In the future, NEC will
promote the development of new power ICs
for USB, such as 1-circuit output products
and 4-circuit products supporting multiple
outputs.
NEC is also studying the feasibility of
developing a power IC for the IEEE1394
high-speed serial interface.
In this way, NEC plans to continuously
develop new products that conform to new
products and new standards in the PC
market.
Example: PC
5-V Power Supply
Pull-Up Resistor
USB Port1
5V
Control1
Flag1
Control2
Flag2
Output1
Input
1µF
GND
Output2
100µF
USB Port2
5V
D+
D-
GND
100µF
D+
D-
GND
Power IC for USB
USB
Controller
ASIC
µPD16855
Signal Line (pair)
Fig. 6 Example of USB System (PC)
Test/Measurement
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