LM2704 Datasheet, PDF (8/11 Pages) National Semiconductor (TI) – Micropower Step-up DC/DC Converter with 550mA Peak Current Limit

Operation (Continued)
The LM2704 features a constant off-time control scheme.
Operation can be best understood by referring to Figure 2
and Figure 3. Transistors Q1 and Q2 and resistors R3 and
R4 of Figure 2 form a bandgap reference used to control the
output voltage. When the voltage at the FB pin is less than
1.237V, the Enable Comp in Figure 2 enables the device and
the NMOS switch is turned on pulling the SW pin to ground.
When the NMOS switch is on, current begins to flow through
inductor L while the load current is supplied by the output
capacitor C_OUT. Once the current in the inductor reaches the
peak current limit, the CL Comp trips and the 400ns One
Shot turns off the NMOS switch. The SW voltage will then
rise to the output voltage plus a diode drop and the inductor
current will begin to decrease as shown in Figure 3. During
this time the energy stored in the inductor is transferred to
C_OUT and the load. After 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
Figure 3.
This cycle is continued until the voltage at FB reaches
1.237V. When FB reaches this voltage, the enable compara-
tor 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_OUT indicated by the gradually
decreasing slope at the output as shown in Figure 3. When
the FB pin drops slightly below 1.237V, the enable compara-
tor enables the device and begins the cycle described pre-
viously. The SHDN pin can be used to turn off the LM2704
and reduce the I_q to 0.01µA. In shutdown mode the output
voltage will be a diode drop lower than the input voltage.
Application Information
INDUCTOR SELECTION
The appropriate inductor for a given application is calculated
using the following equation:
where V_D is the schottky diode voltage, I_CL is the switch
current limit found in the Typical Performance Characteris-
tics section, and T_OFF is the switch off time. When using this
equation be sure to use the minimum input voltage for the
application, such as for battery powered applications. For
the LM2704 constant-off time control scheme, the NMOS
power switch is turned off when the current limit is reached.
There is approximately a 200ns 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
200ns delay, the peak inductor current will increase. This
increase in inductor current demands a larger saturation
current rating for the inductor. This saturation current can be
approximated by the following equation:
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
voltage to 5V, the 400ns off time may not be enough time to
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
ripple on the output voltage. Using a smaller inductor will
cause the I_PK to increase and will increase the output voltage
ripple further. This can be solved by adding a 4.7pF capaci-
tor across the R_F1 feedback resistor (Figure 2) and slightly
increasing the output capacitor. A smaller inductor can then
be used to ensure proper discharge in the 400ns off time.
DIODE SELECTION
To maintain high efficiency, the average current rating of the
schottky diode should be larger than the peak inductor cur-
rent, I_PK. Schottky diodes with a low forward drop and fast
switching speeds are ideal for increasing efficiency in por-
table 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
sufficient. For some applications a reduction in output volt-
age ripple can be achieved by increasing the output capaci-
tor.
Local bypassing for the input is needed on the LM2704.
Multilayer ceramic capacitors are a good choice for this as
well. A 4.7µF capacitor is sufficient for most applications. For
additional bypassing, a 100nF ceramic capacitor can be
used to shunt high frequency ripple on the input.
LAYOUT CONSIDERATIONS
The input bypass capacitor C_IN, as shown in Figure 1, must
be placed close to the IC. This will reduce copper trace
resistance which effects input voltage ripple of the IC. For
additional input voltage filtering, a 100nF bypass capacitor
can be placed in parallel with C_IN to shunt any high fre-
quency noise to ground. The output capacitor, C_OUT, should
also be placed close to the IC. Any copper trace connections
for the Cout capacitor can increase the series resistance,
which directly 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 feed-
back 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 di-
rectly to the GND pin. Trace connections made to the induc-
tor and schottky diode should be minimized to reduce power
dissipation and increase overall efficiency.
Choosing inductors with low ESR decrease power losses
and increase efficiency.
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