tON =
V
IN
1.3 x 10
-10
x
R1
1 Introduction
The LM2695EVAL evaluation board provides the design engineer with a fully functional buck regulator,
employing the constant on-time (COT) operating principle. This evaluation board provides a 10 V output
over an input range of 12 V - 30 V. The circuit delivers load currents to 1A, with current limit set at ≊1.3A.
The board is populated with all external components except R5, C8 and C11. These components provide
options for changing the current limit threshold, and managing the output ripple as described later in this
document.
The board’s specification are:
• Input Voltage: 12 V to 30 V
• Output Voltage: 10 V
• Maximum load current: 1.0A
• Minimum load current: 0A
• Current Limit: 1.3A
• Measured Efficiency: 96.3% (VIN= 12 V, I
• Nominal Switching Frequency: 380 kHz
• Size: 2.25 in. x 0.88 in. x 0.47 in
User's Guide
SNVA147A–February 2006–Revised April 2013
AN-1444 LM2695 Evaluation Board
= 300 mA)
OUT
2 Theory of Operation
Figure 6 shows a simplified block diagram of the LM2695. When the circuit is in regulation, the buck
switch is on each cycle for a time determined by R1 and VINaccording to Equation 1:
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SNVA147A–February 2006–Revised April 2013 AN-1444 LM2695 Evaluation Board
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Figure 1. Evaluation Board - Top Side
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1
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Board Layout and Probing
The on-time of this evaluation board ranges from ≊2300 ns at VIN= 12 V, to ≊900 ns at VIN= 30 V. The
on-time varies inversely with VINto maintain a nearly constant switching frequency. At the end of each ontime the Minimum Off-Timer ensures the buck switch is off for at least 250 ns. In normal operation, the offtime is much longer. During the off-time, the output capacitor (C7) is discharged by the load current. When
the output voltage falls sufficiently that the voltage at FB is below 2.5 V, the regulation comparator initiates
a new on-time period. For stable, fixed frequency operation, ≊25 mV of ripple is required at FB to switch
the regulation comparator. For a more detailed block diagram and a complete description of the various
functional blocks, see the LM2695 High Voltage (30V, 1.25A) Step Down Switching Regulator Data Sheet
(SNVS413).
3 Board Layout and Probing
The pictorial in Figure 1 shows the placement of the circuit components. The following should be kept in
mind when the board is powered:
• When operating at high input voltage and high load current, forced air flow is recommended.
• The LM2695, and diode D1 may be hot to the touch when operating at high input voltage and high load
current.
• Use CAUTION when probing the circuit at high input voltages to prevent injury, as well as possible
damage to the circuit.
• At maximum load current (1A), the wire size and length used to connect the load becomes important.
Ensure there is not a significant drop in the wires between this evaluation board and the load.
4 Board Connection/Start-up
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The input connections are made to the J1 connector. The load is normally connected to the OUT1 and
GND terminals of the J3 connector. Ensure the wires are adequately sized for the intended load current.
Before start-up a voltmeter should be connected to the input terminals, and to the output terminals. The
load current should be monitored with an ammeter or a current probe. It is recommended that the input
voltage be increased gradually to 12 V, at which time the output voltage should be 10 V. If the output
voltage is correct with 12 V at VIN, then increase the input voltage as desired and proceed with evaluating
the circuit.
5 Output Ripple Control
The LM2695 requires a minimum of 25 mVp-p ripple at the FB pin, in phase with the swtiching waveform
at the SW pin, for proper operation. In the simplest configuration that ripple is derived from the ripple at
V
, generated by the inductor’s ripple current flowing through R4. That ripple voltage is attenuated by
OUT1
the feedback resistors, requiring that the ripple amplitude at V
p by the gain factor. Options for reducing the output ripple are discussed below, and the results are shown
in the graph of Figure 9.
5.1 Minimum Output Ripple
This evaluation board is configured for minimum ripple at V
components R6, C9 and C10. The output ripple that ranges from 3mVp-p at VIN= 12 V to 8 mVp-p at VIN=
30 V is determined primarily by the ESR of output capacitor (C7), and the inductor’s ripple current that
ranges from 50 mAp-p to 195 mAp-p over the input voltage range. The ripple voltage required by the FB
pin is generated by R6, C9 and C10 since the SW pin switches from -1 V to VIN, and the right end of C9 is
a virtual ground. The values for R6 and C9 are chosen to generate a 30-40 mVp-p triangle waveform at
their junction. That triangle wave is then coupled to the FB pin through C10. The following procedure is
used to calculate values for R6, C9 and C10:
• Calculate the voltage VAas shown in Equation 2:
VA= V
where, VSWis the absolute value of the voltage at the SW pin during the off-time (typically 1 V) and V
is the minimum input voltage. For this circuit, VAcalculates to 9.83 V. This is the DC voltage at the
R6/C9 junction, and is used in Equation 3.
- (VSWx (1 - (V
OUT
be higher than the minimum of 25 mVp-
OUT1
by setting R4 to 0 Ω, and including
OUT1
))) (2)
OUT/VIN
IN
2
AN-1444 LM2695 Evaluation Board SNVA147A–February 2006–Revised April 2013
Copyright © 2006–2013, Texas Instruments Incorporated
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FB
SW
LM2695
BST
VCC
C6
D1
ISEN
SGND
VIN
SS
RTN
10V
GND
C3
13
11
10
9
6
5
4
2
3
12
GND
C1 C2
2.2 PF
2.2
PF
C5
0.022 PF
0.1
PF
0.022 PF
C4
0.1 PF
L1 100 PH
RON/SD
R1
200k
R2
7.5k
R3
2.5k
R4
0
C7
22 PF
C10
0.01 PF
R6
165k
C9
1000 pF
Input
V
OUT2
V
OUT1
R6 x C9 =
0.03V
(12V ± 9.83V) x 2300 ns
= 1.66 x 10
-4
R6 x C9 =
'V
(VIN ± VA) x t
ON
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• Calculate the R6 x C9 product as shown in Equation 3:
R6 and C9 are then chosen from standard value components to satisfy the above product. For example,
C9 can be 1000 pF requiring R6 to be 166 kΩ. C10 is chosen to be 0.01 µF, large compared to C9. The
circuit as supplied on this EVB is shown in Figure 2.
Output Ripple Control
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where tONis the maximum on-time (≊2300 ns), VINis the minimum input voltage, and ΔV is the desired
ripple amplitude at the R6/C9 junction, 30 mVp-p for this example.
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Figure 2. Minimum Ripple Using R6, C9, C10
SNVA147A–February 2006–Revised April 2013 AN-1444 LM2695 Evaluation Board
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3
C8 =
(R2//R3)
t
ON(max)
FB
SW
LM2695
BST
VCC
C6
D1
ISEN
SGND
VIN
SS
RTN
10V
GND
C3
13
11
10
9
6
5
4
2
3
12
GND
C1 C2
2.2 PF
2.2
PF
C5
0.022 PF
0.1
PF
0.022 PF
C4
0.1 PF
L1 100 PH
RON/SD
R1
200k
R2
7.5k
R3
2.5k
R4
0.55:
C7
22 PF
C8
1200 pF
Input
V
OUT2
V
OUT1
Output Ripple Control
5.2 Intermediate Ripple Level Configuration
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This configuration generates more ripple at V
than the above configuration, but uses one less
OUT1
capacitor. If some ripple can be tolerated in the application, this configuration is slightly more economical,
and simpler. R4 and C8 are used instead of R6, C9, and C10, as shown in Figure 3.
Figure 3. Intermediate Ripple Level Configuration Using C8 and R4
R4 is chosen to generate ≥25 mV - 30 mVp-p at V
, knowing that the minimum ripple current in this
OUT1
circuit is 50 mAp-p at minimum VIN. C8 couples that ripple to the FB pin without the attenuation of the
feedback resistors. C8's minimum value is calculated from Equation 5:
where t
is the maximum on-time (at minimum VIN), and R2//R3 is the equivalent parallel value of the
ON(max)
feedback resistors. For this evaluation board t
C8 calculates to a minimum of 1200 pF. The resulting ripple at V
over the input voltage range.
4
AN-1444 LM2695 Evaluation Board SNVA147A–February 2006–Revised April 2013
is approximately 2300 ns, and R2//R3 = 1.875 kΩ, and
ON(max)
OUT1
Copyright © 2006–2013, Texas Instruments Incorporated
ranges from 27 mVp-p to 105 mVp-p
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