LM25010EVAL National Semiconductor, LM25010EVAL Datasheet - Page 2

• Manufacturer Part Number: LM25010EVAL
• Description: BOARD EVALUATION LM25010
• Manufacturer: National Semiconductor

Specifications of LM25010EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 6 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 200kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25010
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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Board Connection/Start-up
ammeter or a current probe. It is recommended that the input
voltage be increased gradually to 6V, at which time the
output voltage should be 5V. If the output voltage is correct
with 6V at V
and proceed with evaluating the circuit.
Reducing Output Ripple
The LM25010 requires a minimum of 25 mVp-p ripple at the
FB pin, in phase with the switching waveform at the SW pin,
for proper operation. In the basic application circuit shown in
the data sheet, C8 is not included. The required ripple at FB
is derived from the ripple at V
inductor’s ripple current passing through R4 and the ESR of
capacitor C7. Since the ripple voltage at V
by the R2/R3 feedback divider, a minimum of 50 mVp-p is
required at V
intended application, C8 can be removed from this evalua-
tion board, and R4 increased to 1.5Ω. In that case, the
minimum ripple amplitude ()55 mVp-p) occurs at minimum
Vin (6V), and increases to )300 mVp-p at Vin = 42V, as
shown in Figure 6.
If a low ripple output is desired three alternatives are de-
scribed below.
A) Ripple Reduction Option A: This EVB is supplied with
C8 installed, and R4 = 0.68Ω, providing a relatively low ripple
output at V
FB without attenuation. The ripple amplitude at V
ranges from 30mVp-p to 150 mVp-p (see Figure 6) as Vin is
varied over its range. The minimum value for C8 is calcu-
lated from:
where t
R2//R3 is the equivalent parallel value of R2 and R3. For this
The resulting ripple at V
6V, to 11 mVp-p at Vin = 42V, and varies slightly with load
current. See Figure 6. These values are valid only for con-
tinuous conduction mode (load current is between 120 mA
ON(max)
OUT1
IN
OUT1
, then increase the input voltage as desired
is the maximum on-time at minimum Vin, and
since C8 couples the output ripple directly to
. If this ripple level is acceptable for the
OUT1
OUT1
ranges from 3 mVp-p at Vin =
, which is generated by the
FIGURE 2. Low Ripple Output Using R6, C9, C10
OUT1
is attenuated
(Continued)
OUT1
2
evaluation board, t
R2//R3 = 2.5kΩ, resulting in a minimum value of 0.002 µF for
C8.
B) Ripple Reduction Option B: Add R6, C9, C10, replace
R4 with zero ohms, and leave C8 and C11 positions open.
Since the SW pin switches from -1V to V
of C9 is a virtual ground, R6 and C9 are chosen to generate
a 30-40 mVp-p triangle wave at their junction. That triangle
wave is coupled to the FB pin through C10. To calculate the
values for R6, C9, and C10, use the following procedure,
using the minimum input voltage for Vin:
Calculate the voltage V
where V
during the off-time (typically 1V), and Vin is the minimum
input voltage. For this circuit, V
the DC voltage at the R6/C9 junction, and is used in the next
equation.
Calculate the R6 • C9 product:
where t
the desired ripple amplitude at the R6/C9 junction, 30mV for
this example.
R6 and C9 are then chosen from standard value compo-
nents to satisfy the above product. For example, C9 can be
1000 pF, requiring R6 to be 195 kΩ. C10 is chosen to be
0.01 µF, large compared to C9. The resulting circuit is:
and 1.3A). If the load current is reduced below 120 mA such
that the circuit operates in discontinuous conduction mode
ON
SW
is the on-time at minimum Vin ()5 µs), and ∆V is
V
is the absolute value of the voltage at the SW pin
A
= V
OUT
ON(max)
- (V
A
:
SW
is approximately 5000 ns, and
x (1 - (V
A
20176006
calculates to 4.83V. This is
OUT
IN
, and the right end
/Vin)))