Texas Instruments AN-1870 LM26420 User manual

User's Guide
SNVA353B–February 2009–Revised April 2013
AN-1870 LM26420 Evaluation Board
1 Introduction
The LM26420 evaluation board was designed to provide two 2A outputs, VOUT1 and VOUT2 . It is available in
either the HTSSOP package option of the LM26420 for easier probing or the WQFN version for evaluating
the smaller package. The design emphasizes on the compactness of the LM26420 PCB layout and is
thermally optimized. The total solution size is less than 35mm by 40mm. The board supports the
conversion from an input voltage ranging from 3V to 5.5V down to output voltages of 0.8V for each
channel. VOUT1 and VOUT2 are set to 1.2V and 2.5V respectively. The feedback resistor can be changed to
support output voltages as low as 0.8V or as high at 4.5V, assuming VIN is high enough to support it. The
EN pins are pulled up to VIN by jumpers for easy evaluation but can also be easily controlled by external
logic.
The board’s specifications are:
• Input Voltage: 3V to 5.5V
• Output Voltages: 1.2VOUT1 and 2.5VOUT2
• Maximum load current: 2A/output
• Minimum load current: 0A
• Size: 1.35 in. × 1.6 in.
• Peak Current Limit: ≊3.2A at 25°C
• Nominal Switching Frequency: 550 kHz or 2.2MHz
2 Powering Up the Board
Since the EN pins are directly tied to the input voltage via jumpers J1and J2, starting up the board is as
simple as connecting a voltage supply from 3V to 5.5V between the VIN and GND terminals. There should
be 1.2V on VOUT1 and 2.5V on VOUT2, assuming the jumpers connect VIN to EN for each channel. Great
care should be taken in powering up the supplies such that the input voltage, VIN, does not exceed the
Absolute Maximum Rating of 7V. If the part experiences voltages greater than 7V for a prolonged period
of time, then damage to the part can occur and then the evaluation board may cease working.
The linear soft-start ramps for the two output voltages and should last about 600 µs. Load can be applied
prior to power-up. 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.
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Typical Application Circuit www.ti.com
3 Typical Application Circuit
Figure 1. LM26420 Demo Board Schematic
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www.ti.com 16-Pin WQFN Pin Out
4 16-Pin WQFN Pin Out
Figure 2. 16-Pin WQFN (top view)
Table 1. Pin Descriptions 16-Pin WQFN
Pin Name Function
1,2 VIND1Power Input supply for Buck1.
11, 12 VIND2Power Input supply for Buck2.
15 VINC Power Input supply for control circuitry.
4 PGND1Power ground pin for Buck 1.
9 PGND2Power ground pin for Buck 2.
14 AGND Signal ground pin. Place the bottom resistor of the feedback network as close as possible to pin.
6 PG1Power Good Indicator for Buck 1. Pin is connected through a resistor to an external supply (open
collector output).
7 PG2Power Good Indicator for Buck 2. Pin is connected through a resistor to an external supply (open
collector output).
5 FB1Feedback pin for Buck 1. Connect to external resistor divider to set output voltage.
8 FB2Feedback pin for Buck 2. Connect to external resistor divider to set output voltage.
3 SW1Output switch for Buck 1. Connect to the inductor.
10 SW2Output switch for Buck 2. Connect to the inductor.
16 EN1Enable control input. Logic high enable operation for Buck 1. Do not allow this pin to float or be
greater than VIN + 0.3V.
13 EN2Enable control input. Logic high enable operation for Buck 2. Do not allow this pin to float or be
greater than VIN + 0.3V.
DAP Die Attach Pad Connect to system ground for low thermal impedance and as a primary electrical GND
connection.
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20-Pin HTSSOP Pin Out www.ti.com
5 20-Pin HTSSOP Pin Out
Figure 3. 20-Pin HTSSOP (top view)
Table 2. Pin Descriptions 20-Pin HTSSOP
Pin Name Function
3, 4 VIND1Power Input supply for Buck1.
17, 18 VIND2Power Input supply for Buck2.
1 VINC Power Input supply for control circuitry.
6,7 PGND1Power ground pin for Buck 1.
14, 15 PGND2Power ground pin for Buck 2.
20 AGND Signal ground pin. Place the bottom resistor of the feedback network as close as possible to pin.
9 PG1Power Good Indicator for Buck 1. Pin is connected through a resistor to an external supply (open
drain output).
12 PG2Power Good Indicator for Buck 2. Pin is connected through a resistor to an external supply (open
drain output).
8 FB1Feedback pin for Buck 1. Connect to external resistor divider to set output voltage.
13 FB2Feedback pin for Buck 2. Connect to external resistor divider to set output voltage.
5 SW1Output switch for Buck 1. Connect to the inductor.
16 SW2Output switch for Buck 2. Connect to the inductor.
2 EN1Enable control input. Logic high enable operation for Buck 1. Do not allow this pin to float or be
greater than VIN + 0.3V.
19 EN2Enable control input. Logic high enable operation for Buck 2. Do not allow this pin to float or be
greater than VIN + 0.3V.
10,11 NC No Connect.
DAP Die Attach Pad Connect to system ground for low thermal impedance, but it cannot be used as a primary GND
connection.
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www.ti.com Schematic - X Version
6 Schematic - X Version
Figure 4. Schematic - X Version
Table 3. Bill of Materials - X Version
Item Designator Description Manufacturer Part No. Qty.
C1, C2, C3,
1 22µF, 6.3V, X5R, 1206 TDK C3216X5R0J226(M or K) 6
C4, C6, C7
2 C5 0.47µF, 16V, X7R, 0603 TDK C2012X7R1C474K 1
SPM6530T-1R0M120 or
3 L1, L2 1µH, 6.4A TDK 2
RLF7030T-1R0N6R4
4 R1 5.1k, 1%, 603 Vishay CRCW06035K10F 1
5 R2 21.3k, 1%, 603 Vishay CRCW060321K3F 1
6 R7, R8 49.9k, 1%, 603 Vishay CRCW060349K9F 2
R3, R4, R5,
7 10k, 1%, 603 Vishay CRCW060310K0F 4
R6
8 R9 5.1 Ohm, 1%, 805 Vishay CRCW06035R10F 1
9 J1, J2 Jumper for Enables Sullins Connector Solutions STC02SYAN 2
0.094" Diameter Solder
10 TP1-TP8 Keystone Electronics 5011 8
Terminal
Dual 2A, 2.2MHz PWM
11 U1 Switcher, HTSSOP-20 or Texas Instruments LM26420 or LM26420 1
WQFN-16
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Schematic - Y Version www.ti.com
7 Schematic - Y Version
Figure 5. Schematic - Y Version
Table 4. Bill of Materials - Y Version
Item Designator Description Manufacturer Part No. Qty.
C1, C2, C3,
1 22µF, 6.3V, X5R, 1206 TDK C3216X5R0J226(M or K) 6
C4, C6, C7
2 C5 0.47µF, 16V, X7R, 0603 TDK TMK105BJ104KV-F 1
RLF7030T-3R3M4R1 or
3 L1, L2 3.3µH, 6.4A TDK or Coilcraft 2
MSS7341-332NL_
4 R1 5.1k, 1%, 603 Vishay CRCW06035K10F 1
5 R2 21.3k, 1%, 603 Vishay CRCW060321K3F 1
6 R7, R8 49.9k, 1%, 603 Vishay CRCW060349K9F 2
R3, R4, R5,
7 10k, 1%, 603 Vishay CRCW060310K0F 4
R6
8 R9 5.1 Ohm, 1%, 805 Vishay CRCW06035R10F 1
9 J1, J2 Jumper for Enables Sullins Connector Solutions STC02SYAN 2
0.094" Diameter Solder
10 TP1-TP8 Keystone Electronics 5011 8
Terminal
Dual 2A, 550kHz PWM LM26420YMH or
11 U1 Switcher, HTSSOP-20 or Texas Instruments 1
LM26420YSQ
WQFN-16
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www.ti.com HTSSOP-20 PCB Layout
8 HTSSOP-20 PCB Layout
HTSSOP - Top Layer HTSSOP - Layer 2
HTSSOP - Layer 3 HTSSOP - Bottom Layer
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WQFN-16 PCB Layout www.ti.com
9 WQFN-16 PCB Layout
WQFN - Top Layer WQFN - Layer 2
WQFN - Layer 3 WQFN - Bottom Layer
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www.ti.com Typical Performance Characteristics
10 Typical Performance Characteristics
Efficiency - 1.2VOUT - X Version Efficiency - 2.5VOUT - X Version
Efficiency - 1.2VOUT - Y Version Efficiency - 2.5VOUT - Y Version
Load Step Response - X Version Load Step Response - Y Version
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Typical Performance Characteristics www.ti.com
Steady State Operation - Y Version Enable
Start-up - VIN Applied
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