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| 型号： | LTC1938IDD#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 4 A SWITCHING REGULATOR, 3300 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件页数： | 6/24页 |
| 文件大小： | 272K |
| 代理商： | LTC1938IDD#TRPBF |
LT1938
14
1938f
Loop compensation determines the stability and transient
performance. Designing the compensation network is a
bit complicated and the best values depend on the ap-
plication 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.
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 LT1938 control loop.
The error amplier is a transconductance amplier with
nite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplier generating an output cur-
rent 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 ampli-
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 RC 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
Figure 3. Transient Load Response of the LT1938 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. VOUT = 3.3V
Figure 2. Model for Loop Response
APPLICATIONS INFORMATION
–
+
1.265V
SW
VC
GND
3M
LT1938
1938 F02
R1
OUTPUT
ESR
CF
CC
RC
ERROR
AMPLIFIER
FB
R2
C1
CURRENT MODE
POWER STAGE
gm = 3.5mho
gm =
330
μmho
+
POLYMER
OR
TANTALUM
CERAMIC
CPL
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 efciency. 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. The circuit in Figure 4a
is more efcient because the BOOST pin current and BIAS
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BIAS pins are not exceeded.
The minimum operating voltage of an LT1938 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
1938 F03
IL
1A/DIV
VOUT
100mV/DIV
10
μs/DIV
VOUT = 12V, FRONT PAGE APPLICATION
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