LM34910EVAL National Semiconductor, LM34910EVAL Datasheet - Page 7

• 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: -

Start-up Regulator, V
The start-up regulator is integral to the LM34910. The input
pin (V
with transient capability to 40V. The V
7.0V, and is current limited to 9 mA. Upon power up, the
regulator sources current into the external capacitor at V
(C3). When the voltage on the V
voltage lockout threshold of 5.8V, the buck switch is enabled
and the Softstart pin is released to allow the Softstart capaci-
tor (C6) to charge up.
The minimum input voltage is determined by the regulator’s
dropout voltage, the V
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the
Softstart pin (2.5V). In normal operation (the output voltage
is regulated), an on-time period is initiated when the voltage
at FB falls below 2.5V. The buck switch stays on for the
on-time, causing the FB voltage to rise above 2.5V. After the
on-time period, the buck switch stays off until the FB voltage
falls below 2.5V. Bias current at the FB pin is nominally 100
nA.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.875V refer-
ence. If the voltage at FB rises above 2.875V the on-time
pulse is immediately terminated. This condition can occur if
the input voltage or the output load changes suddenly, or if
the inductor (L1) saturates. The buck switch remains off until
the voltage at FB falls below 2.5V.
ON-Time Timer, and Shutdown
The on-time for the LM34910 is determined by the R
resistor and the input voltage (V
IN
) can be connected directly to line voltage up to 36V,
CC
UVLO falling threshold ()5.7V),
IN
CC
), and is calculated from:
CC
pin reaches the under-
CC
output regulates at
FIGURE 5. Self Biased Configuration
CC
ON
7
and the frequency. When V
old the V
externally loaded, the minimum input voltage increases
since the output impedance at V
To reduce power dissipation in the start-up regulator, an
auxiliary voltage can be diode connected to the V
Setting the auxiliary voltage to between 8V and 14V shuts off
the internal regulator, reducing internal power dissipation.
The sum of the auxiliary voltage and the input voltage (V
+ V
to V
See Figure 3. The inverse relationship with V
nearly constant frequency as V
selected for a minimum on-time (at maximum V
than 200 ns. This requirement limits the maximum frequency
for each application, depending on V
from the following:
The LM34910 can be remotely shut down by taking the
R
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the R
mal operation to resume. The voltage at the R
between 1.5V and 3.0V, depending on V
resistor.
ON
IN
IN
/SD pin below 0.65V. See Figure 6. In this mode the SS
) cannot exceed 50V. Internally, a diode connects V
. See Figure 5.
CC
UVLO activates to shut off the output. If V
CC
20110911
falls below the falling thresh-
IN
CC
is varied. R
is )140Ω. See Figure 2.
IN
ON
and V
/SD pin allows nor-
IN
OUT
IN
ON
and the R
ON
, calculated
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results in a
IN
should be
/SD pin is
) greater
CC
CC
pin.
(4)
(5)
ON
CC
CC
is