LM25005_06 NSC [National Semiconductor], LM25005_06 Datasheet - Page 10

• Manufacturer Part Number: LM25005_06
• Description: 42V, 2.5A Step-Down Switching Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
Shutdown / Standby
The LM25005 contains a dual level Shutdown (SD) circuit.
When the SD pin voltage is below 0.7V, the regulator is in a
low current shutdown mode. When the SD pin voltage is
greater than 0.7V but less than 1.225V, the regulator is in
standby mode. In standby mode the Vcc regulator is active
but the output switch is disabled. When the SD pin voltage
exceeds 1.225V, the output switch is enabled and normal op-
eration begins. An internal 5µA pull-up current source config-
ures the regulator to be fully operational if the SD pin is left
open.
An external set-point voltage divider from Vin to GND can be
used to set the operational input range of the regulator. The
divider must be designed such that the voltage at the SD pin
will be greater than 1.225V when Vin is in the desired oper-
ating range. The internal 5µA pull-up current source must be
included in calculations of the external set-point divider. Hys-
teresis of 0.1V is included for both the shutdown and standby
thresholds. The voltage at the SD pin should never exceed
8V. When using an external set-point divider, it may be nec-
essary to clamp the SD pin to limit its voltage at high input
voltage conditions.
The SD pin can also be used to implement various remote
enable / disable functions. Pulling the UVLO pin below the
0.7V threshold totally disables the controller. If the SD pin
voltage is above 1.225V the regulator will be operational.
Oscillator and Sync Capability
The LM25005 oscillator frequency is set by a single external
resistor connected between the RT pin and the AGND pin.
The R
connected directly to the pins of the IC (RT and AGND).To
set a desired oscillator frequency (F), the necessary value for
the R
The SYNC pin can be used to synchronize the internal oscil-
lator to an external clock. The external clock must be of
higher frequency than the free-running frequency set by the
R
recommended interface from the external clock to the SYNC
pin. The clock pulse duration should be greater than 15 ns.
T
resistor. A clock circuit with an open drain output is the
T
T
resistor can be calculated from the following equation:
resistor should be located very close to the device and
FIGURE 3. Sync from External Clock
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Multiple LM25005 and/or LM5005 devices can be synchro-
nized together simply by connecting the SYNC pins together.
In this configuration all of the devices will be synchronized to
the highest frequency device. The diagram in Figure 5 illus-
trates the SYNC input/output features of the LM25005. The
internal oscillator circuit drives the SYNC pin with a strong
pull-down / weak pull-up inverter. When the SYNC pin is
pulled low either by the internal oscillator or an external clock,
the ramp cycle of the oscillator is terminated and a new os-
cillator cycle begins. Thus, if the SYNC pins of several
LM25005 IC’s are connected together, the IC with the highest
internal clock frequency will pull the connected SYNC pins low
first and terminate the oscillator ramp cycles of the other IC’s.
The LM25005 or LM5005 with the highest programmed clock
frequency will serve as the master and control the switching
frequency of the all the devices with lower oscillator frequen-
cy.
FIGURE 4. Sync from Multiple Devices
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