LT1940/LT1940L
U
W U U
APPLICATIO S I FOR ATIO
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 3V, as in
Figure 3d. The diode is used to prevent damage to the
LT1940 in case VIN2 is held low while VIN is present. The
circuit saves several components (both BOOST pins can
be tied to D2). However, efficiency may be lower and
dissipation in the LT1940 may be higher. Also, if VIN2 is
absent, theLT1940willstillattempttoregulatetheoutput,
but will do so with very low efficiency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
of the loop compensation but is used to filter 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 appli-
cation 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
compensation network to optimize the performance. Sta-
bility should then be checked across all operating condi-
tions, including load current, input voltage and tempera-
ture. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
The minimum input voltage of an LT1940 application is
limited by the minimum operating voltage (<3.6V) 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
LT1940 is turned on with its RUN/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
runningproperly. Thisminimumloadwilldependoninput
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. The Typical Performance Characteris-
tics 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 capaci-
tor will present a load to the switcher which will allow it to
start. The plots show the worst-case situation where VIN
is ramping very slowly. Use a Schottky diode (such as the
BAT-54) for the lowest start-up voltage.
Figure 4 shows an equivalent circuit for the LT1940
control loop. The error amp is a transconductance ampli-
fier with finite output impedance. The power section,
consistingofthemodulator, powerswitchandinductor, is
modeled as a transconductance amplifier generating an
output current proportional to the voltage at the VC pin.
Note that the output capacitor integrates this current, and
that the capacitor on the VC pin (CC) integrates the error
amplifier 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 CC.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (CPL) across the feedback divider may improve
the transient response.
LT1940
CURRENT MODE
POWER STAGE
m
V
SW
Frequency Compensation
OUTPUT
ERROR
g
= 2.5mho
C
R1
AMPLIFIER
PL
The LT1940 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT1940 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.
FB
–
g
=
m
340µmho
ESR
+
500k
1.25V
C1
+
GND
V
C
C1
POLYMER
OR
TANTALUM
CERAMIC
R2
R
C
Frequency compensation is provided by the components
tied to the VC pin. Generally a capacitor and a resistor in
series to ground determine loop gain. In addition, there is
alowervaluecapacitorinparallel.Thiscapacitorisnotpart
C
F
C
C
1940 F05
Figure 4. Model for Loop Response
1940fa
12
Linear [ Linear ]
