lt1940l Linear Technology Corporation, lt1940l Datasheet - Page 12

• Manufacturer Part Number: lt1940l
• Description: Lt1940l - Dual Monolithic 1.4a, 1.1mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT1940/LT1940L
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 V
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 V
absent, the LT1940 will still attempt to regulate the output,
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.
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
running properly. This minimum load will depend on input
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 V
is ramping very slowly. Use a Schottky diode (such as the
BAT-54) for the lowest start-up voltage.
Frequency Compensation
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.
Frequency compensation is provided by the components
tied to the V
series to ground determine loop gain. In addition, there is
a lower value capacitor in parallel. This capacitor is not part
12
APPLICATIO S I FOR ATIO
C
pin. Generally a capacitor and a resistor in
IN2
U
is held low while V
U
W
IN
is present. The
U
IN2
is
IN
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.
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,
consisting of the modulator, power switch and inductor, is
modeled as a transconductance amplifier generating an
output current proportional to the voltage at the V
Note that the output capacitor integrates this current, and
that the capacitor on the V
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 C
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 (C
the transient response.
LT1940
GND
CURRENT MODE
POWER STAGE
g
m
= 2.5mho
V
R
C
C
C
500k
C
PL
Figure 4. Model for Loop Response
) across the feedback divider may improve
340 mho
C
g
AMPLIFIER
m
F
ERROR
=
–
+
V
1.25V
SW
FB
C
pin (C
R2
R1
C
TANTALUM
) integrates the error
POLYMER
C
PL
C1
ESR
OR
+
OUTPUT
C1
CERAMIC
1940 F05
C
1940fa
pin.
C
.