LT1766 Linear Technology, LT1766 Datasheet - Page 21

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
V
skipping or odd/even cycle behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
2. The simultaneous requirements of high V
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response, the following should be remembered—the worse
the board layout, the more difficult the circuit will be to
stabilize. This is true of almost all high frequency analog
circuits, read the Layout Considerations section first.
Common layout errors that appear as stability problems
are distant placement of input decoupling capacitor and/
or catch diode, and connecting the V
ground track carrying significant switch current. In addi-
tion, the theoretical analysis considers only first order
non-ideal component behavior. For these reasons, it is
important that a final stability check is made with produc-
tion layout and components.
The LT1766 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 10.
The LT1766 can be considered as two g
amplifier and the power stage.
Figure 11 shows the overall loop response. At the V
the frequency compensation components used are:
R
capacitor used is a 100 F, 10V tantalum capacitor with
typical ESR of 100m .
The ESR of the tantalum output capacitor provides a useful
zero in the loop frequency response for maintaining stabil-
C
C
V
practice due to internal dissipation. The Thermal Con-
siderations section offers a basis to estimate internal
power. In questionable cases a prototype supply should
be built and exercised to verify acceptable operation.
and high f
minimum switch on time. Cycle skipping and/or odd/
even cycle behavior will result although correct output
voltage is usually maintained.
control voltage to the point where some sort of cycle-
= 2.2k, C
IN
, high I
OUT
C
OSC
= 0.022 F and C
and high f
can result in an unacceptably short
U
U
OSC
may not be achievable in
F
W
= 220pF. The output
C
compensation to a
m
blocks, the error
IN
U
, low V
C
pin,
OUT
ity. This ESR, however, contributes significantly to the
ripple voltage at the output (see Output Ripple Voltage in
the Applications Section). It is possible to reduce capaci-
tor size and output ripple voltage by replacing the tantalum
output capacitor with a ceramic output capacitor because
of its very low ESR. The zero provided by the tantalum
output capacitor must now be reinserted back into the
loop. Alternatively there may be cases where, even with
the tantalum output capacitor, an additional zero is re-
quired in the loop to increase phase margin for improved
transient response.
A zero can be added into the loop by placing a resistor, R
at the V
or by placing a capacitor, CFB, between the output and the
FB pin.
LT1766
GND
CURRENT MODE
POWER STAGE
g
m
= 2mho
C
V
R
C
pin in series with the compensation capacitor, C
C
C
200k
C
R
O
Figure 10. Model for Loop Response
2000 mho
Figure 11. Overall Loop Response
–20
–40
80
60
40
20
g
C
AMPLIFIER
0
m
F
ERROR
10
=
V
V
I
C
PHASE
LOAD
–
+
IN
OUT
OUT
= 42V
= 5V
= 100 F, 10V, 0.1
= 500mA
100
1.22V
V
SW
GAIN
FB
FREQUENCY (Hz)
LT1766/LT1766-5
1k
C
FB
10k
R
C
C
R
R1
R2
C
C
F
LOAD
= 220pF
= 22nF
= 2.2k
100k
1766 F10
+
1766 F11
1M
TANTALUM
ESR
C1
180
150
120
90
60
30
0
OUTPUT
21
CERAMIC
1766fa
ESL
C1
C,
C