Texas Instruments LM34904EVM Evaluation Module LM34904EVM/NOPB LM34904EVM/NOPB Scheda Tecnica
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LM34904EVM/NOPB
User's Guide
SNVU156A – July 2012 – Revised May 2013
AN-2266 LM34902/4 Current Limited Power Switch
Evaluation Module
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Introduction
With the advent of new handset, tablet, and notebook devices in smaller and smaller footprints, engineers
are facing the challenging task of designing next-generation systems that fit into these smaller form
factors. Also, consumers are placing more value on the battery life of their portable devices by seeking
longer operation time between charges.
are facing the challenging task of designing next-generation systems that fit into these smaller form
factors. Also, consumers are placing more value on the battery life of their portable devices by seeking
longer operation time between charges.
One of the standards that has become a “must have” for these devices is the USB port that enables a
user to connect various accessories to their device. However, power transfer from a host device to a
downstream accessory needs to be carefully managed to avoid inrush current and/or voltage transient
events upon connection. The accessory typically sends a handshake signal to indicate that it is ready to
accept downstream power transfer. Only then should the device’s CPU or system controller authorize
current flow. Well-defined overload protection is also mandated to reliably manage capacitive loads or
accessory fault modes.
user to connect various accessories to their device. However, power transfer from a host device to a
downstream accessory needs to be carefully managed to avoid inrush current and/or voltage transient
events upon connection. The accessory typically sends a handshake signal to indicate that it is ready to
accept downstream power transfer. Only then should the device’s CPU or system controller authorize
current flow. Well-defined overload protection is also mandated to reliably manage capacitive loads or
accessory fault modes.
With nominal 5 nA shutdown and 47 µA quiescent supply currents, the LM34902/4 current-limited power
switch is ideally suited for such applications. Offered in a tiny 6-bump 1.2 mm x 0.8 mm micro SMD
package with 0.6 mm height and 0.4 mm pin pitch, the LM34902 and LM34904 are rated for 300 mA and
500 mA continuous load currents, respectively. An input voltage range of 2.8V to 5.3V is provided to cater
to 3V and 5V systems. With a low ON resistance, the high-side p-channel MOSFET switch power
dissipation is minimized, translating into lower temperature increase—a critical feature in space
constrained applications—and longer battery life. Using an integrated sense resistor, the LM34902/4
provides accurate brickwall current limit, protecting the input supply even when the ACC_PWR output is
hard-shorted to ground (GND). To assure reliability, thermal shutdown protection is provided if the junction
temperature exceeds 135°C (LM34904) or 170°C (LM34902).
switch is ideally suited for such applications. Offered in a tiny 6-bump 1.2 mm x 0.8 mm micro SMD
package with 0.6 mm height and 0.4 mm pin pitch, the LM34902 and LM34904 are rated for 300 mA and
500 mA continuous load currents, respectively. An input voltage range of 2.8V to 5.3V is provided to cater
to 3V and 5V systems. With a low ON resistance, the high-side p-channel MOSFET switch power
dissipation is minimized, translating into lower temperature increase—a critical feature in space
constrained applications—and longer battery life. Using an integrated sense resistor, the LM34902/4
provides accurate brickwall current limit, protecting the input supply even when the ACC_PWR output is
hard-shorted to ground (GND). To assure reliability, thermal shutdown protection is provided if the junction
temperature exceeds 135°C (LM34904) or 170°C (LM34902).
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Evaluation Module Features
•
Input voltage operating range: 2.8V to 5.3V.
•
Ambient temperature range: -40°C to +85°C.
•
Rated load current: 300 mA (LM34902), 500 mA (LM34904).
•
PCB size: 1.7” × 1.7” (43 mm × 43 mm).
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Evaluation Module Operation
As the pinouts and footprints of the LM34902 and LM34904 are identical, the solution presented in this
user's guide enables the user to evaluate both power switches. This commonality provides flexibility if the
load current specifications evolve during the design process. The schematic in
user's guide enables the user to evaluate both power switches. This commonality provides flexibility if the
load current specifications evolve during the design process. The schematic in
demonstrates how
the LM34902/4 switch connects to a device CPU (or an ASIC with embedded µC core) and an accessory
load. Presented also is the LM34902/4 truth table operation (
load. Presented also is the LM34902/4 truth table operation (
When the accessory is connected, it asserts an active-low accessory detect (ACC_DET) control input. The
CPU can then selectively turn on (or off) the switch using the ENABLE input, providing power to the
accessory (or conserving power when needed). Both the ENABLE and ACC_DET inputs are 1.8V logic
compatible. Both inputs need to be asserted to turn on the MOSFET switch (ACC_DET first and then
ENABLE). Note that the power switch is not bidirectional: the input voltage must therefore be higher than
the output voltage under all conditions.
CPU can then selectively turn on (or off) the switch using the ENABLE input, providing power to the
accessory (or conserving power when needed). Both the ENABLE and ACC_DET inputs are 1.8V logic
compatible. Both inputs need to be asserted to turn on the MOSFET switch (ACC_DET first and then
ENABLE). Note that the power switch is not bidirectional: the input voltage must therefore be higher than
the output voltage under all conditions.
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SNVU156A – July 2012 – Revised May 2013
AN-2266 LM34902/4 Current Limited Power Switch Evaluation Module
Copyright © 2012–2013, Texas Instruments Incorporated