lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 16

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1941
and is largest when V
second, lower power channel draws input current, the
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. The ripple current contribution
from the third channel will be minimal. Considering that
the maximum load current from a single channel is ~2.8A,
RMS ripple current will always be less than 1.4A.
The high frequency of the LT1941 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is often less than 10µF. The combi-
nation of small size and low impedance (low equivalent
series resistance or ESR) of ceramic capacitors makes
them the preferred choice. The low ESR results in very low
voltage ripple. Ceramic capacitors can handle larger mag-
nitudes of ripple current than other capacitor types of the
same value. Use X5R and X7R types.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for ex-
ample a 1µF ceramic capacitor in parallel with a low ESR
tantalum capacitor. For the electrolytic capacitor, a value
larger than 10µF will be required to meet the ESR and
ripple current requirements. Because the input capacitor
is likely to see high surge currents when the input source
is applied, tantalum capacitors should be surge rated. The
manufacturer may also recommend operation below the
rated voltage of the capacitor. Be sure to place the 1µF
ceramic as close as possible to the V
the IC for optimal noise immunity.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plug-
ging the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT1941. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Frequency Compensation
The LT1941 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
16
U
IN
= 2 V
U
OUT
(50% duty cycle). As the
W
IN
and GND pins on
U
LT1941 does not depend on the ESR of the output capaci-
tor for stability so you are free to use ceramic capacitors
to achieve low output ripple and small circuit size.
The components tied to the V
compensation. Generally, a capacitor and a resistor in
series to ground determine loop gain. In addition, there is
a lower value capacitor in parallel. This capacitor filters
noise at the switching frequency and is not part of the loop
compensation.
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 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. Check stability across
all operating conditions, including load current, input volt-
age and temperature. The LT1375 data sheet contains a
more thorough discussion of loop compensation and de-
scribes how to test the stability using a transient load.
Application Note 76 is an excellent source as well.
Figure 5 shows an equivalent circuit for the LT1941
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 either from the
output capacitor ESR or from a resistor in series with C
This model works well as long as the inductor current
ripple is not too low (∆I
crossover frequency is less than ƒ
capacitor (C
the transient response.
The equivalent circuit for the LT1941 inverter control loop
is slightly different than is shown in Figure 5. The feedback
resistors are connected as shown for negative outputs in
Figure 2. The operational amplifier is fast enough to have
minimal effect on the loop dynamics.
PL
) across the feedback divider may improve
RIPPLE
C
pin (C
> 5% I
C
pin provide frequency
C
) integrates the error
SW
OUT
/5. A phase lead
) and the loop
C
1941fa
pin.
C
.