LT1936 Linear Technology, LT1936 Datasheet - Page 10

• Manufacturer Part Number: LT1936
• Description: 1.4A 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT1936
This is the minimum output capacitance required, not
the nominal capacitor value. For example, a 3.3V output
requires 20μF of output capacitance. If a small 22μF , 6.3V
ceramic capacitor is used, the circuit may be unstable be-
cause the effective capacitance is lower than the nominal
capacitance when biased at 3.3V. Look carefully at the
capacitor’s data sheet to fi nd out what the actual capaci-
tance is under operating conditions (applied voltage and
temperature). A physically larger capacitor, or one with a
higher voltage rating, may be required.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specifi ed by the supplier, and should be 0.05Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
Frequency Compensation
The LT1936 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT1936 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
tor (C
In addition, there may be lower value capacitor in parallel.
10
C
) and a resistor (R
C
pin, as shown in Figure 1. Generally a capaci-
Table 2. Capacitor Vendors
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
C
) in series to ground are used.
PHONE
[phone]
[phone]
[phone]
[phone]
[phone]
URL
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
This capacitor (C
but is used to fi lter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR. An alternative to using
external compensation components is to use the internal
RC network by tying the COMP pin to the V
duces component count but does not provide the optimum
transient response when the output capacitor value is high,
and the circuit may not be stable when the output capacitor
value is low. If the internal compensation network is not
used, tie COMP to ground or leave it fl oating.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
LT1936
CURRENT MODE
POWER STAGE
g
C
m
F
= 2mho
PART SERIES
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
V
R
600k
C
C
C
C
Figure 1. Model for Loop Response
COMP
50k
250μmho
F
g
AMPLIFIER
150pF
) is not part of the loop compensation
m
ERROR
=
–
+
GND
1.2V
SW
COMMENTS
EEF Series
T494, T495
POSCAP
TPS Series
FB
R2
R1
www.DataSheet4U.com
TANTALUM
POLYMER
C
PL
ESR
C1
OR
+
C
1936 F01
pin. This re-
OUTPUT
C1
CERAMIC
1936fd