LM25007MM/NOPB National Semiconductor, LM25007MM/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM25007MM/NOPB
• Description: IC REG SW STEPDOWN 0.5A 8-MSOP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM25007MM/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.5 ~ 37 V
• Current - Output: 500mA
• Frequency - Switching: Up to 800kHz
• Voltage - Input: 9 ~ 42 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Primary Input Voltage: 42V
• No. Of Outputs: 1
• Output Voltage: 37V
• Output Current: 500mA
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Input Voltage Primary Max: 42V
• Rohs Compliant: Yes
• For Use With: LM25007EVAL - BOARD EVALUATION LM25007
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM25007MM; LM25007MMTR

Detailed Operating Description
The LM25007 is a monolithic step-down switching regulator
featuring all of the functions needed to implement low cost,
efficient, power supply. Capable of driving a 0.5Amp load over
a 9-42V input voltage range, this device is easy to apply and
is provided in the small MSOP-8 and the thermally enhanced
LLP-8 packages. Ultra-fast transient response is achieved
through the use of a constant on-time architecture with Vin
feed forward. This feature provides and almost constant op-
erating frequency over load and input voltage variations. The
operating frequency may be adjusted up to 800kHz depend-
ing on input and output voltages. This architecture is easy to
use and tolerant of component selection. An intelligent current
limit is implemented in the LM25007 with forced off time that
is inversely proportional to Vout to ensure recovery from fault
conditions. The output voltage may be set from 2.5V to >30V.
To improve efficiency in light load applications, the Vcc pin
can be tied to an external voltage source thus eliminating the
drop through the integrated internal regulator. Additional pro-
tection features include: thermal shutdown, Vcc under-volt-
age lockout, gate drive under-voltage lockout, and max duty
cycle limiter.
Hysteretic Control Circuit Overview
The LM25007 is a Buck DC-DC regulator that uses a constant
on-time control scheme. The on-time is programmed by an
external resistor and varies inversely with line input voltage
(Vin). The core regulation elements of the LM25007 are the
feedback comparator and the on-time one-shot. The regulator
output voltage is sensed at the feedback pin (FB) and is com-
pared to an internal reference voltage (2.5V). If the FB signal
is below the reference voltage, the buck switch is turned on
for a fixed time pulse determined by the line voltage and a
programming resistor (R
switch will remain off for at least the minimum off timer period
of 300ns. If the FB pin voltage is still below the reference after
the 300ns off-time, the switch will turn on again for another on
time period. This switching behavior will continue until the FB
pin voltage reaches the reference voltage level.
The LM25007 operates in discontinuous conduction mode at
light load currents or continuous conduction mode at heavier
load currents. In discontinuous conduction mode, current
through the output inductor starts at zero and ramps up to a
peak value during the buck switch on-time and then back to
zero during the off-time. The inductor current remains at zero
until the next on-time period starts when FB falls below the
internal reference. In discontinuous mode the operating fre-
quency is relatively low and will vary with load. Therefore at
light loads the conversion efficiency is maintained, since the
switching losses decrease with the reduction in load current
and switching frequency. The approximate discontinuous
mode operating frequency can be calculated as follows:
ON
). Following the on period the
7
In continuous conduction mode, current flows continuously
through the inductor and never ramps down to zero. In this
mode the operating frequency is greater than the discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
The output voltage (Vout) can be programmed by two external
resistors as shown in Figure 1. The regulated voltage is cal-
culated as follows:
The feedback comparator in hysteretic regulators depends on
the output ripple voltage to switch the output transistor on and
off at regular intervals. In order for the internal comparator to
respond quickly to changes in output voltage, proportional to
inductor current, a minimum amount of capacitor Equivalent
Series Resistance (ESR) is required. A ripple voltage of 25mV
is recommended at the feedback pin (FB) for stable operation.
In cases where the intrinsic capacitor ESR is too small, addi-
tional series resistance may be added.
For applications where lower output voltage ripple is required
the load can be connected directly to the low ESR output ca-
pacitor, as shown in Figure 1. The series resistor (R5) will
degrade the load regulation. Another technique for enhancing
the ripple voltage at the FB pin is to place a capacitor in par-
allel with the feedback divider resistor R3. The addition of the
capacitor reduces the attenuation of the ripple voltage from
the feedback divider.
High Voltage Bias Regulator (VCC)
The LM25007 contains an internal high voltage bias regulator.
The input pin (Vin) can be connected directly to line voltages
from 9 to 42 Volts. To avoid supply voltage transients due to
long lead inductances on the input pin (VIN), it is always rec-
ommended to connect a low ESR ceramic chip capacitor (
0.1µF) between VIN and RTN pins located close to the
LM25007. The regulator is internally current limited to 11mA.
Upon power up, the regulator is enabled and sources current
into an external capacitor connected to the VCC pin. When
the voltage on the VCC pin reaches its UVLO level (6.3V), the
controller output is enabled.
An external auxiliary supply voltage can be diode connected
to the Vcc pin. If the auxiliary voltage is greater than 7 Volts
the internal regulator will shutoff, thus reducing internal power
dissipation. Do not exceed 14V at VCC. See Figure 2.
V
OUT
= 2.5 x (R3 + R4) / R4
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