LT1912EMSE#PBF Linear Technology, LT1912EMSE#PBF Datasheet - Page 13

• Manufacturer Part Number: LT1912EMSE#PBF
• Description: IC, STEP-DOWN REGULATOR, MSOP-10
• Manufacturer: Linear Technology

Specifications of LT1912EMSE#PBF

• Primary Input Voltage: 36V
• No. Of Outputs: 1
• Output Voltage: 20V
• Output Current: 2A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Supply Voltage Range: 3.6V To 36V
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
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 discussion of
loop compensation and describes how to test the stabil-
ity using a transient load. Figure 2 shows an equivalent
circuit for the LT1912 control loop. The error amplifier is a
transconductance amplifier 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
integrates this current, and that the capacitor on the V
(C
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
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 feedback divider 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 efficiency. 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
another supply greater than 2.8V. Tying BD to V
C
) integrates the error amplifier output current, resulting
C
in series with C
C
pin. Note that the output capacitor
C
. This simple model works
PL
IN
) across
reduces
C
pin
the maximum input voltage to 30V. The circuit in Figure 4a
is more efficient 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,
then the boost capacitor may not be fully charged. Because
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
FB
10µs/DIV
R2
R1
TANTALUM
POLYMER
C
PL
ESR
C1
OR
+
1912 F03
LT1912
1912 F02
OUTPUT
C1
CERAMIC
13
1912fa