LT1939
16
1939f
Generally, for outputs of 3.3V and higher the standard
circuit (Figure 5a) is the best. For outputs between 2.8V
and 3.3V, replace the D2 with a small Schottky diode such
as the PMEG4005.
For lower output voltages the boost diode can be tied to
the input (Figure 5b). The circuit in Figure 5a is more ef-
fi cient because the BST pin current comes from a lower
voltage source.
Figure 5c shows the boost voltage source from the linear
output that is set to greater than 2.5V (any available DC
sources that are greater than 2.5V is suffi cient). The highest
effi ciency is attained by choosing the lowest boost volt-
age above 2.5V. You must also be sure that the maximum
voltage at the BST pin is less than the maximum specifi ed
in the Absolute Maximum Ratings section.
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 2.5V, as
in Figure 5d. The diode is used to prevent damage to the
LT1939 in case V
X
is held low while V
IN
is present. The
circuit eliminates a capacitor, but effi ciency may be lower
and dissipation in the LT1939 may be higher. Also, if V
X
is
absent, the LT1939 will still attempt to regulate the output,
but will do so with very low effi ciency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
The minimum input voltage of an LT1939 application is
limited by the minimum operating voltage (<2.8V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT1939 is turned on with its SS pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
input and output voltages and on the arrangement of the
boost circuit.
The Typical Performance Characteristics section shows
plots of the minimum load current to start and to run as a
function of input voltage for 3.3V and 5V outputs. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
IN
is ramping very slowly.
Use a Schottky diode for the lowest start-up voltage.
Frequency Compensation
The LT1939 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT1939 does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
C
pin. Generally a capacitor and a resistor in series to
ground determine loop gain. In addition, there is a lower
value capacitor in parallel. This capacitor is not part of
the loop compensation but is used to fi lter noise at the
switching frequency.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature.
The LT1375 data sheet contains a more thorough discus-
sion of loop compensation and describes how to test the
stability using a transient load.
Figure 6 shows an equivalent circuit for the LT1939 control
loop. The error amp is a transconductance amplifi er with
fi nite output impedance. The power section, consisting of
the modulator, power switch, and inductor, is modeled as
a transconductance amplifi er generating an output cur-
rent proportional to the voltage at the V
C
pin. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
pin (C
C
) integrates the error ampli-
fi er output current, resulting in two poles in the loop. In
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor in series with C
C
.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
APPLICATIONS INFORMATION
