LM25007EVAL National Semiconductor, LM25007EVAL Datasheet - Page 9

• Manufacturer Part Number: LM25007EVAL
• Description: BOARD EVALUATION LM25007
• Manufacturer: National Semiconductor

Specifications of LM25007EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 450mA
• Voltage - Input: 9 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 306kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25007
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Over-Voltage Comparator
The over-voltage comparator is provided to protect the output
from overvoltage conditions due to sudden input line voltage
changes or output loading changes. The over-voltage com-
parator monitors the FB pin versus an internal 2.875V refer-
ence. If the voltage at FB rises above 2.875V the comparator
immediately terminates the buck switch on-time pulse.
ON Time generator and Shutdown
The on-time of the LM25007 is set inversely proportional to
the input voltage by an external resistor connected between
RON/SD and VIN. The RON/SD terminal is a low impedance
input biased at approximately 1.5V. The current through the
resistor and into the RON/SD pin is approximately propor-
Current Limit
The LM25007 contains an intelligent current limit off-timer in-
tended to reduce the foldback characteristic inherent with
fixed off-time over-current protection. If the current in the Buck
switch exceeds 725mA the present cycle on-time is immedi-
ately terminated (cycle by cycle current limit). Following the
termination of the cycle a non-resetable current limit off timer
is initiated. The duration of the off time is a function of the
external resistor (RCL) and the FB pin voltage. When the FB
pin voltage equals zero, the current limit off-time is internally
preset to 17µs. This condition occurs in short circuit operation
when a maximum amount of off-time is required.
In cases of overload (not complete short circuit) the current
limit off-time is reduced as a function of the output voltage
(measured at the FB pin). Reducing the off-time with smaller
overloads reduces the amount of foldback and also reduces
the initial start-up time. The current limit off time for a given
FB pin voltage and R
lowing equation:
Applications utilizing low resistance inductors and/or a low
voltage drop rectifier may require special evaluation at high
line, short circuit conditions. In this special case the preset
Toff = 10
-5
/ (0.59 + (V
CL
resistor can be calculated by the fol-
FB
/ 7.22 x 10
-6
FIGURE 3. Shutdown Implementation
x R
CL
))
9
tional to Vin and used internally to control the on-timer. This
scheme of input voltage feed-forward hysteretic operation
achieves nearly constant operational frequency over varying
line and load conditions. The on-time equation for the
LM25007 is :
The RON/SD pin of the LM25007 also provides a shutdown
function which disables the regulator and significantly de-
creases quiescent power dissipation. By pulling the pin below
0.7V activates the low power shutdown mode. The V
escent current in the shutdown mode is approximately 100µA
internal to the LM25007 plus the current in the R
See Figure 3
17µs (FB = 0V) off-time may be insufficient to balance the
inductor volt*time product. Additional inductor resistance, out-
put resistance or a larger voltage drop rectifier may be nec-
essary to balance the inductor cycle volt*time product and
limit the short circuit current.
N - Channel Buck Switch and Driver
The LM25007 integrates an N-Channel Buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. The bootstrap capac-
itor is charged by V
A 0.01uF ceramic capacitor connected between the BST pin
and SW pin is recommended.
During each cycle when the Buck switch turns off, the SW pin
is approximately 0V. When the SW pin voltage is low, the
bootstrap capacitor is charged from Vcc through the internal
diode. The minimum off timer, set to 300ns, ensures that there
will be a minimum interval every cycle to recharge the boot-
strap capacitor.
An external re-circulating diode from the SW pin to ground is
necessary to carry the inductor current after the internal Buck
switch turns off. This external diode must be of the Ultra-fast
or Schottky type to reduce turn-on losses and current over-
Ton = 1.42 x 10
CC
through the internal high voltage diode.
20165507
-10
x R
ON
/ V
IN
www.national.com
ON
resistor.
IN
qui-