LM34910EVAL National Semiconductor, LM34910EVAL Datasheet - Page 9

• Manufacturer Part Number: LM34910EVAL
• Description: BOARD EVALUATION LM34910
• Manufacturer: National Semiconductor

Specifications of LM34910EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 8 ~ 36V
• Regulator Topology: Buck
• Frequency - Switching: 850kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34910
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Thermal Shutdown
The LM34910 should be operated so the junction tempera-
ture does not exceed 125˚C. If the junction temperature
increases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175˚C, takes the controller to a low power
reset state by disabling the buck switch and the on-timer,
and grounding the Softstart pin. This feature helps prevent
catastrophic failures from accidental device overheating.
When the junction temperature reduces below 155˚C (typical
hysteresis = 20˚C), the Softstart pin is released and normal
operation resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components.
R1 and R2: The ratio of these resistors is calculated from:
R1 and R2 should be chosen from standard value resistors
in the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio.
R
Equation 1 can be used to select R
is desired as long as the above limitation is met.
L1: The main parameter affected by the inductor is the
output current ripple amplitude (I
be determined at both the minimum and maximum nominal
load currents.
a) If the maximum load current is less than the current limit
threshold (1.25A), the minimum load current is used to de-
termine the maximum allowable ripple. To maintain continu-
ous conduction mode the lower peak should not reach 0 mA.
For this case, the maximum ripple current is:
The ripple calculated in Equation 6 is then used in the
following equation:
where V
mined from equation 1. This provides a minimum value for
L1. The next larger standard value should be used, and L1
should be rated for the I
b) If the maximum load current is greater than the current
limit threshold (1.25A), the LM34910 ensures the lower peak
reaches 1.25A each cycle, requiring that I
the difference. The upper peak, however, must not exceed
3.5A. For this case, the ripple limits are:
and
The lesser of Equations 8 and 9 is then used in Equation 7.
If I
The next larger value should then be used for L1. If I
is used, the minimum V
ON
OR(MAX2)
: The minimum value for R
IN
is the maximum input voltage and Fs is deter-
is used, the maximum V
I
I
OR(MIN1)
OR(MAX2)
R1/R2 = (V
I
OR(MAX1)
= 2 x (I
PK
= 2 x (3.5A - I
IN
is used in Equation 7. The next
current level.
= 2 x I
OUT
O(max)
ON
OR
/2.5V) - 1
is calculated from:
ON
). The limits for I
O(min)
IN
- 1.25A)
if a specific frequency
is used in Equation 7.
O(max)
OR
be at least twice
)
OR
OR(MIN1)
must
(6)
(7)
(8)
(9)
9
smaller value should then be used for L1. L1 must be rated
for the peak value of the current waveform (I
C3: The capacitor on the V
filtering and stability, but also prevents false triggering of the
V
reason, C3 should be no smaller than 0.1 µF, and should be
a good quality, low ESR, ceramic capacitor.
C2, and R3: Since the LM34910 requires a minimum of 25
mV
quired ripple at V
necessary ripple is created by the inductor ripple current
acting on C2’s ESR + R3. The minimum ripple current is
calculated using equation 7, rearranged to solve for I
minimum V
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in Figure 1. Generally R3 is less
than 1Ω. C2 should generally be no smaller than 3.3 µF,
although that is dependent on the frequency and the allow-
able ripple amplitude at V
necessary to determine the minimum value for C2, as the
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for C2
than a non-varying load.
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck
switch is turned on. The forward voltage drop is significant in
the event the output is short-circuited as it is mainly this
diode’s voltage (plus the voltage across the current limit
sense resistor) which forces the inductor current to decrease
during the off-time. For this reason, a higher voltage is better,
although that affects efficiency. A reverse recovery time of
)30 ns, and a forward voltage drop of )0.75V are preferred.
The reverse leakage specification is important as that can
significantly affect efficiency. D1’s reverse voltage rating
must be at least as great as the maximum V
current rating must equal or exceed I
C1 and C5: C1’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at V
on the assumption that the voltage source feeding V
an output impedance greater than zero. If the source’s dy-
namic impedance is high (effectively a current source), it
supplies the average input current, but not the ripple current.
At maximum load current, when the buck switch turns on, the
current into V
inductor’s ripple current, ramps up to the peak value, then
drop to zero at turn-off. The average current during the
on-time is the load current. For a worst case calculation, C1
must supply this average load current during the maximum
on-time. C1 is calculated from:
where Io is the load current, t
and ∆V is the allowable ripple voltage at V
to help avoid transients and ringing due to long lead induc-
tance at V
recommended, located close to the LM34910 .
CC
p-p
UVLO at the buck switch on/off transitions. For this
of ripple at the FB pin for proper operation, the re-
IN
IN
. A low ESR, 0.1 µF ceramic chip capacitor is
. The minimum ESR for C2 is then equal to:
IN
suddenly increases to the lower peak of the
OUT1
is increased by R1 and R2. This
CC
OUT1
output provides not only noise
ON
. Experimentation is usually
is the maximum on-time,
PK
Figure 7.
IN
. C5’s purpose is
PK
in Figure 7).
www.national.com
IN
, and its
IN
OR
(10)
has
IN
at
,