User's Guide
SNVU156A–July 2012–Revised May 2013
AN-2266 LM34902/4 Current Limited Power Switch
Evaluation Module
1 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.
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.
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).
2 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).
3 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 Figure 1 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 (Table 1).
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.
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ENABLE
ACC_PWR
GND
VCC
POK ACC_DET
A1
B1
C1
A2
B2
C2
LM34902/4
CPU Accessory
200k
Accessory
Detect
VCC: 2.8V to 5.3V
200k
Evaluation Module Operation
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Figure 1. Typical System Schematic
Table 1. Power Switch Truth Table
Input (1) Output
Current Limit TJLimit PFET Switch
ENABLE ACC_DET 2.8V < VCC < 5.3V POK
Detected Exceeded Status
0 x No No Yes Open Open Drain
x 1 No No Yes Open Open Drain
0 to 1 0 No No Yes On Grounded
0 to 1 0 Yes No Yes Current Grounded
Limited
x x x Yes 2.2V < VCC < 5.3V Open Open Drain
0 x x No 2.2V < VCC < 2.8V Open Open Drain
(1) x = stands for "don't care".
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ACC_PWR
GND
VCC
ACC_DET
A1
B1
C1
A2
B2
C2
LM34902/4
R1
POK
R2
C1 C2
R4
ENABLE
C3
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Evaluation Module Schematic
4 Evaluation Module Schematic
The schematic of the evaluation module is shown in Figure 2. Note that ENABLE and ACC_DET default to
their off-states if left open-circuited in this implementation. An active-low open-drain output (POK) provides
an indication of switch status to the CPU. As shown in Figure 2, the module provides pullup resistors to
VCC for POK and ACC_DET and a pulldown resistor to GND for ENABLE.
Figure 2. Evaluation Module Schematic
5 Evaluation Module Bill of Materials
Designator Value Description Manufacturer Part Number Qty
C1, C2 22 µF Ceramic, 22µF, X5R, 10V, 10%, 0805 Taiyo Yuden LMK212BJ226MG-T 2
C3 4.7 µF Ceramic, 4.7µF, X5R, 25V, 10%, Murata GRM21BR61E475KA12L 1
0805
VCC, Test Point, TH, Miniature, Red Keystone Electronics 5000 2
ACC_PWR
ENABLE, Test Point, TH, Miniature, White Keystone Electronics 5002 3
ACC_DET,
POK
GND Test Point, TH, Miniature, Black Keystone Electronics 5001 1
R1, R2, R4 220k Resistor, 220k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603220KFKEA 3
U1 500mA Current Limited Power Switch Texas Instruments LM34904 1
3
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Evaluation Module Component Selection
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6 Evaluation Module Component Selection
Ceramic input and output capacitors are positioned close to the load switch. Specifically, two 22 µF
bypass capacitors from VCC to GND minimize input voltage drop during a high current startup event or
momentary output short circuit condition. A 1 µF capacitor is usually adequate even though larger
capacitance will further reduce the voltage drop. A 4.7 µF decoupling capacitor from ACC_PWR to GND
alleviates any negative swing of ACC_PWR induced by parasitic inductance in the load lines as the load
current ramps to zero during a turn-off event. Of course, all traces short be kept as short as possible with
VCC, ACC_PWR and GND power bus traces kept wide to minimize conduction drop and assist with
thermal spreading.
When the output is short-circuited, the input voltage appears across the switch, and the instantaneous
power dissipation can be quite high. If the fault condition is not removed, the die temperature may
increase to the level where the thermal shutdown protection activates. If the faulted short-circuit condition
persists and the device engages thermal shutdown, it latches off. ENABLE can be cycled to initiate a re-
start.
7 Evaluation Module Power Up Procedure
Step 1: Apply an input voltage between 2.8V and 5.3V (VCC to GND).
Step 2: Connect ACC_DET to GND.
Step 3: Connect ENABLE to VCC. The load switch turns on – measure voltage from ACC_PWR to GND.
Step 4: Apply a load current up to the rated current of the device – check voltage from ACC_PWR to
GND.
Step 5: Apply and remove an overload or short circuit – confirm recovery from this condition.
Step 6: Cycle ENABLE high and low to assess turn on and turn off wavefroms, respectively.
8 Evaluation Module Performance Characteristics
Using the LM34904 with ACC_DET and POK pullup resistors connected to a pullup voltage source of
1.8V, various performance characteristics of the evaluation module are presented in Figure 3 through
Figure 9.
Figure 3. ENABLE Turn On, VCC = 2.8V, IOUT = 0.5A Figure 4. ENABLE Turn On, VCC = 5.3V, IOUT = 0.5A
(Resistive Load) (Resistive Load)
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Evaluation Module Performance Characteristics
Figure 5. ENABLE Turn Off, VCC = 2.8V, IOUT = 0.5A Figure 6. ENABLE Turn Off, VCC = 5.3V, IOUT = 0.5A
(Resistive Load) (Resistive Load)
Figure 7. ENABLE Turn On Into An Overload Figure 8. Output Transient Response When Cycling
Condition (4ΩResisitve Load), VCC = 5.3V Into and Out Of Current Limit (Electronic Load
Output), VCC = 5.3V
Figure 9. ENABLE and ACC_DET Cycled Independently, VCC = 5.3V
5
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PCB Layout
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9 PCB Layout
The LM34902/4 evaluation module uses a two-layer FR4 PCB with terminal connections provided for
VCC, ACC_PWR, GND, ENABLE, ACC_DET and POK. The top and bottom side PCB layouts are shown
in Figure 10 and Figure 11, respectively.
Figure 10. PCB Top Layer
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