LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 13

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
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.
Stability should then be checked across all operating
conditions, including load current, input voltage and
temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Figure 2
shows an equivalent circuit for the LT1912 control loop.
The error amplifi er 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
the output capacitor integrates this current, and that the
capacitor on the V
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 R
C
of the inductor is not too high and the loop crossover
frequency is much lower than the switching frequency.
A phase lead capacitor (C
may improve the transient response. Figure 3 shows the
transient response when the load current is stepped from
500mA to 1500mA and back to 500mA.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.22μF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best effi ciency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 4b). For lower output voltages the
boost diode can be tied to the input (Figure 4c), or to
C
. This simple model works well as long as the value
C
pin (C
PL
C
) across the feedback divider
) integrates the error ampli-
C
C
pin. Note that
in series with
another supply greater than 2.8V. Tying BD to V
the maximum input voltage to 30V. The circuit in Figure 4a
is more effi cient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT1912 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
Figure 3. Transient Load Response of the LT1912 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. V
LT1912
CURRENT MODE
POWER STAGE
g
m
C
100mV/DIV
F
= 3.5mho
0.5A/DIV
V
R
3Meg
C
V
C
C
OUT
OUT
C
Figure 2. Model for Loop Response
I
L
= 3.3V
V
420μmho
IN
g
AMPLIFIER
= 12V; FRONT PAGE APPLICATION
m
ERROR
=
–
+
GND
0.8V
SW
10μs/DIV
FB
R2
R1
TANTALUM
POLYMER
C
PL
C1
ESR
OR
+
1912 F03
LT1912
1912 F02
OUTPUT
IN
C1
CERAMIC
reduces
13
1912f