LM2705MF-ADJ/NOPB National Semiconductor, LM2705MF-ADJ/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM2705MF-ADJ/NOPB
• Description: IC CONV DC/DC STEP-UP SOT23-5
• Manufacturer: National Semiconductor

Specifications of LM2705MF-ADJ/NOPB

• Constant Current: *
• Constant Voltage: *
• Topology: *
• Number Of Outputs: *
• Internal Driver: *
• Type - Primary: *
• Type - Secondary: *
• Frequency: *
• Voltage - Supply: *
• Voltage - Output: *
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Operating Temperature: *
• Current - Output / Channel: *
• Internal Switch(s): *
• Efficiency: *
• Primary Input Voltage: 7V
• No. Of Outputs: 1
• Output Voltage: 20V
• Output Current: 150mA
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Termination Type: SMD
• Rohs Compliant: Yes
• Filter Terminals: SMD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM2705MF-ADJ; LM2705MF-ADJTR

The LM2705 features a constant off-time control scheme.
Operation can be best understood by referring to
Figure
Figure 2
voltage. When the voltage at the FB pin is less than 1.237V,
the Enable Comp in
NMOS switch is turned on pulling the SW pin to ground. When
the NMOS switch is on, current begins to flow through induc-
tor L while the load current is supplied by the output capacitor
C
limit, the CL Comp trips and the 400ns One Shot turns off the
NMOS switch.The SW voltage will then rise to the output volt-
age plus a diode drop and the inductor current will begin to
decrease as shown in
stored in the inductor is transferred to C
the 400ns off-time the NMOS switch is turned on and energy
is stored in the inductor again. This energy transfer from the
inductor to the output causes a stepping effect in the output
ripple as shown in
This cycle is continued until the voltage at FB reaches 1.237V.
When FB reaches this voltage, the enable comparator then
disables the device turning off the NMOS switch and reducing
the Iq of the device to 40uA. The load current is then supplied
solely by C
the output as shown in
slightly below 1.237V, the enable comparator enables the de-
vice and begins the cycle described previously. The SHDN
pin can be used to turn off the LM2705 and reduce the I
0.01µA. In shutdown mode the output voltage will be a diode
drop lower than the input voltage.
Application Information
INDUCTOR SELECTION - BOOST REGULATOR
The appropriate inductor for a given application is calculated
using the following equation:
where V
rent limit found in the Typical Performance Characteristics
section, and T
tion be sure to use the minimum input voltage for the appli-
cation, such as for battery powered applications. For the
LM2705 constant-off time control scheme, the NMOS power
switch is turned off when the current limit is reached. There is
approximately a 100ns delay from the time the current limit is
reached in the NMOS power switch and when the internal
logic actually turns off the switch. During this 100ns delay, the
peak inductor current will increase. This increase in inductor
current demands a larger saturation current rating for the in-
ductor. This saturation current can be approximated by the
following equation:
Choosing inductors with low ESR decrease power losses and
increase efficiency.
Care should be taken when choosing an inductor. For appli-
cations that require an input voltage that approaches the
output voltage, such as when converting a Li-Ion battery volt-
age to 5V, the 400ns off time may not be enough time to
OUT
. Once the current in the inductor reaches the current
3. Transistors Q1 and Q2 and resistors R3 and R4 of
D
form a bandgap reference used to control the output
is the schottky diode voltage, I
OUT
OFF
indicated by the gradually decreasing slope at
is the switch off time. When using this equa-
Figure
Figure 2
Figure
Figure
3.
3. During this time the energy
enables the device and the
3. When the FB pin drops
OUT
CL
and the load. After
is the switch cur-
Figure 2
and
q
to
7
discharge the energy in the inductor and transfer the energy
to the output capacitor and load. This can cause a ramping
effect in the inductor current waveform and an increased rip-
ple on the output voltage. Using a smaller inductor will cause
the I
further.
For typical curves and evaluation purposes the DT1608C se-
ries inductors from Coilcraft were used. Other acceptable
inductors would include, but are not limited to, the SLF6020T
series from TDK, the NP05D series from Taiyo Yuden, the
CDRH4D18 series from Sumida, and the P1166 series from
Pulse.
INDUCTOR SELECTION - SEPIC REGULATOR
The following equation can be used to calculate the approxi-
mate inductor value for a SEPIC regulator:
The boost inductor, L1, can be smaller or larger but is gener-
ally chosen to be the same value as L2. See
Figure 10
DIODE SELECTION
To maintain high efficiency, the average current rating of the
schottky diode should be larger than the peak inductor cur-
rent, I
switching speeds are ideal for increasing efficiency in portable
applications. Choose a reverse breakdown of the schottky
diode larger than the output voltage.
CAPACITOR SELECTION
Choose low ESR capacitors for the output to minimize output
voltage ripple. Multilayer ceramic capacitors are the best
choice. For most applications, a 1µF ceramic capacitor is suf-
ficient. For some applications a reduction in output voltage
ripple can be achieved by increasing the output capacitor.
Output voltage ripple can further be reduced by adding a
4.7pF feed-forward capacitor in the feedback network placed
in parallel with RF1, see
Local bypassing for the input is needed on the LM2705. Mul-
tilayer ceramic capacitors are a good choice for this as well.
A 4.7µF capacitor is sufficient for most applications. For ad-
ditional bypassing, a 100nF ceramic capacitor can be used to
shunt high frequency ripple on the input.
LAYOUT CONSIDERATIONS
The input bypass capacitor C
be placed close to the IC. This will reduce copper trace re-
sistance which effects input voltage ripple of the IC. For
additional input voltage filtering, a 100nF bypass capacitor
can be placed in parallel with C
noise to ground. The output capacitor, C
placed close to the IC. Any copper trace connections for the
Cout capacitor can increase the series resistance, which di-
rectly effects output voltage ripple. The feedback network,
resistors R1 and R2, should be kept close to the FB pin to
minimize copper trace connections that can inject noise into
the system. The ground connection for the feedback resistor
network should connect directly to an analog ground plane.
The analog ground plane should tie directly to the GND pin.
If no analog ground plane is available, the ground connection
for the feedback network should tie directly to the GND pin.
Trace connections made to the inductor and schottky diode
PK
PK
to increase and will increase the output voltage ripple
. Schottky diodes with a low forward drop and fast
for typical SEPIC applications.
Figure
IN
IN
, as shown in
2.
to shunt any high frequency
OUT
, should also be
Figure
Figure
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