LT1767 Linear Technology, LT1767 Datasheet - Page 13

• Manufacturer Part Number: LT1767
• Description: Monolithic 1.5A/ 1.25MHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
Notice that the catch diode’s forward voltage contributes
a significant loss in the overall system efficiency. A larger,
lower V
Typical thermal resistance of the board is 35 C/W. At an
ambient temperature of 65 C,
If a true die temperature is required, a measurement of the
SYNC to GND pin resistance can be used. The SYNC pin
resistance across temperature must first be calibrated,
with no device power, in an oven. The same measurement
can then be used in operation to indicate the die tempera-
ture.
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 prob-
lems are distant placement of input decoupling capacitor
and/or catch diode, and connecting the V
to a ground track carrying significant switch current. In
addition, 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 LT1767 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 7,
with both tantalum and ceramic capacitor equivalent cir-
cuits. The LT1767 can be considered as two g
error amplifier and the power stage.
Figure 8 shows the overall loop response with a 330pF V
capacitor and a typical 100 F tantalum output capacitor.
The response is set by the following terms:
P
L
P
T
DCR
j
INDUCTOR
INDUCTOR
= 65 + 40 (0.4) + 35 (0.39) = 95 C
F
= Inductor DC resistance (assume 0.1 )
diode can improve efficiency by several percent.
= (I
= (1) (0.1) = 0.1W
LOAD
U
) (L
INFORMATION
DCR
U
)
W
C
compensation
m
blocks, the
U
C
LT1767-2.5/LT1767-3.3/LT1767-5
Error amplifier:
DC gain set by g
Pole set by C
Unity-gain set by C
410kHz.
Power stage:
DC gain set by g
Pole set by C
Unity-gain set by C
3.98kHz.
Tantalum output capacitor:
Zero set by C
LT1767
GND
CURRENT MODE
POWER STAGE
g
m
= 2.5mho
–20
–40
V
R
80
60
40
20
C
0
C
C
OUT
10
500k
C
Figure 7. Model for Loop Response
OUT
F
Figure 8. Overall Loop Response
and R
m
and C
850 mho
and R
100
C
and R
g
AMPLIFIER
F
OUT
m
F
ERROR
m
=
and g
LT1767/LT1767-1.8/
L
–
+
ESR
FREQUENCY (Hz)
and R
= (2 • 500k • 330p)
and g
L
L
1k
= (2 • 100 • 10)
= (2 • 100 • 0.1)
V
(assume 10 ) = 2.5 • 10 = 25.
1.2V
SW
m
FB
V
C
C
R
I
LOAD
= (2 • 330p • 850
m
OUT
OUT
C
C
L
/C
10k
= 330pF
= (2 • 100 • 2.5
F
= 5V
= 100 F, 0.1
= 850 • 500k = 425.
= 500mA
= N/C
R1
R2
100k
+
PHASE
GAIN
TANTALUM
ESR
1767 F10
C1
OUTPUT
1M
–1
180
150
120
90
60
30
0
–1
–1
= 15.9kHz.
sn1767 1767fas
CERAMIC
= 965Hz.
= 159Hz.
13
–1
–1
1767 F07
ESL
C1
)
)
–1
–1
=
=