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LM26400Y Demonstration
Board
National Semiconductor
Application Note 1600
Dongbing Zhang
March 2007
Introduction
The LM26400Y demonstration board was designed to provide two 2A outputs at 1.2V and 2.5V respectively. It uses the
ETSSOP package option of the LM26400Y for easier probing.
The design emphasizes on the compactness of the
LM26400Y PCB layout and is thermally optimized. The total
solution size is about 20mm by 30mm. The board supports
the conversion from an input voltage ranging from 5V to 20V
down to output voltages of 1.2V and 2.5V. With the availability
of a separate 5V rail and a couple of small bootstrap diodes,
the board also supports an input voltage down to 3.3V with
2A output currents. The EN pins are pulled up to VIN by default for easy evaluation but can also be easily controlled by
external logic.
The board also has two small CFF capacitors (C12 and C13)
installed for improved load step response and elimination of
output voltage overshoot after a short-circuit release.
The board’s specifications are:
•
Input Voltage: 5V to 20V
•
Output Voltages: 1.2V and 2.5V
•
Maximum load current: 2A/output
•
Minimum load current: 0A
•
Peak Current Limit: ≊ 3A at 25°C
•
Measured Efficiency: 83% (VIN = 5V, I
OUT2
= 2A)
•
Nominal Switching Frequency: 520 kHz
•
Size: 2 in. x 2 in.
Powering Up The Board
Since the EN pins are directly tied to the input voltage, starting
up the board is a single-step procedure. Simply connect a
voltage rail between 5V and 20V to the VIN and GND termi-
nals and there should be 1.2V and 2.5V output at the corresponding terminals. Certain bench-top power supplies upon
powering up may shoot up to their maximum output voltages
momentarily before settling to the programmed value. If their
maximum voltage is above 22V, it can damage the LM26400Y
demonstration board. In this case, either connect the board
after the input power supply is powered up, or use the current
limit knob of the power supply to bring up the input voltage.
The linear soft-start ramps for the two output voltages should
last about 1ms and 2ms. Load can be applied prior to powerup. If no load is applied, the two channels will operate in pulse
skipping mode or discontinuous conduction mode. If an output is shorted either before or after start-up, removal of the
short-circuit condition should bring the corresponding output
back to normal voltage.
If additional output capacitors are desired, C8 and C9 on the
back of the board are reserved for that purpose.
If it is desired to control the start-up and shutdown timing,
connect the logic signals to the EN1 and/or EN2 pads on the
back of the board. Make sure the voltages on the EN pads
are never higher than VIN. If only a soft-start slope needs to
be adjusted, simply change the corresponding SS capacitor
(C5 or C6).
To operate between 3.3V and 5V of input voltage, populate
D3 and D4 (on the back of the board) each with a SOT-23
Schottky diode such as the BAT54 and apply a 5V supply to
the "5V_Bias" pad on the back. Do not exceed 6V on the 5V
bias. The LM26400Y device itself can work with an input voltage as low as 3V. The demonstration board when using an
external bootstrap bias can operate down to 3.3V under room
temperature. This extra 0.3V requirement is due to the large
duty cycle in the 2.5V channel being too close to the maximum
allowed.
© 2007 National Semiconductor Corporation 300127 www.national.com
LM26400Y Demonstration Board AN-1600
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Board Schematic
30012702
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AN-1600
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Bill of Materials
Item Designator Description Manufacturer Part No. Qty.
1 C5, C6, C12, C13 0.022µF, 6.3V, X5R, 10%, 0402 Kemet C0402C223K9PACTU 4
2 C2, C3, C7 0.1µF, 25V, X5R, 10%, 0402 Taiyo Yuden TMK105BJ104KV-F 3
3 C1 10µF, 25V, X5R, 20%, 1210 Taiyo Yuden TMK325BJ106MM-T 1
4 C11 47µF, 6.3V, X5R, 20%, 1210 Taiyo Yuden JMK325BJ476MM-T 1
5 C4 100µF, 6.3V, X5R, 20%, 1210 Taiyo Yuden JMK325BJ107MM-T 1
6 D1, D2 2A, 30V, SMB IR 20BQ030TRPBF 2
7 L1
5µH, 2.2A, 23mΩ, 7x7x2.8mm
3
Sumida CDRH6D26NP-5R0NC 1
8 L2
8.7µH, 2.2A, 25mΩ, 7x7x4mm
3
Sumida CDRH6D38NP-8R7NC 1
9 R1
4.7Ω, 1%, 0402
Vishay CRCW04024R70FNED 1
10 R2, R3, R5
5.9kΩ, 1%, 0402
Vishay CRCW040259R0FKED 3
11 R4, R6, R7
18.7kΩ, 1%, 0402
Vishay CRCW0402187RFKED 3
12 TP1-TP6 0.094" Diameter Solder Terminal Cambion 160-1026-02-01-00 6
13 U1
Dual 2A, 20V, 500kHz PWM Switcher,
ETSSOP-16
National LM26400YMH 1
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AN-1600
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PCB Layout
Top Copper
30012703
Ground Plane
30012704
Power Plane
30012705
Bottom Copper
30012706
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Top Silkscreen
30012707
Bottom Silkscreen
30012708
Top Soldermask
30012709
Bottom Soldermask
30012710
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Typical Performance Characteristics
Efficiency
30012716
Efficiency
30012715
Start-up
30012711
Steady State (Ch1=DCM, Ch2=Pulse Skipping)
30012712
Steady State (Ch1=CCM, Ch2=DCM)
30012713
Load Step Response (slew rate = 0.25A/µs)
30012714
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Notes
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Notes
AN-1600 LM26400Y Demonstration Board
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