LT1767 Linear Technology, LT1767 Datasheet - Page 8

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

LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
AVX TPS, Sprague 593D
AVX TAJ
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
Figure 3 shows a comparison of output ripple for a ceramic
and tantalum capacitor at 200mA ripple current.
INDUCTOR CHOICE AND MAXIMUM OUTPUT
CURRENT
Maximum output current for a buck converter is equal to
the maximum switch rating (I
peak inductor current. In past designs, the maximum
switch current has been reduced by the introduction of
slope compensation. Slope compensation is required at
duty cycles above 50% to prevent an affect called
subharmonic oscillation (see Application Note 19 for
details). The LT1767 has a new circuit technique that
maintains a constant switch current rating at all duty
cycles. (Patent Pending)
For most applications, the output inductor will be in the
1 H to 10 H range. Lower values are chosen to reduce the
physical size of the inductor, higher values allow higher
output currents due to reduced peak to peak ripple current,
8
V
TANTALUM CAPACITOR
OUT
CERAMIC CAPACITOR
USING 47 F, 0.1
V
OUT
USING 2.2 F
(10mV/DIV)
(10mV/DIV)
(5V/DIV)
Figure 3. Output Ripple Voltage Waveform
V
SW
U
INFORMATION
ESR (Max,
U
0.1 to 0.3
0.7 to 0.9
0.1 to 0.3
0.2 (typ)
P
) minus one half peak to
0.2 s/DIV
W
)
Ripple Current (A)
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
0.4
U
1767 F03
and reduces the current at which discontinuous operation
occurs. The following formula gives maximum output
current for continuous mode operation, implying that the
peak to peak ripple (2x the term on the right) is less than
the maximum switch current.
Discontinuous operation occurs when
For V
Note that the worst case (minimum output current avail-
able) condition is at the maximum input voltage. For the
same circuit at 15V, maximum output current would be
only 1.1A.
When choosing an inductor, consider maximum load
current, core and copper losses, allowable component
height, output voltage ripple, EMI, fault current in the
inductor, saturation, and of course, cost. The following
procedure is suggested as a way of handling these some-
what complicated and conflicting requirements.
1. Choose a value in microhenries from the graphs of
I
I
I
Continuous Mode
maximum load current. Choosing a small inductor with
lighter loads may result in discontinuous mode of
operation, but the LT1767 is designed to work well in
either mode.
Assume that the average inductor current is equal to
load current and decide whether or not the inductor
must withstand continuous fault conditions. If maxi-
mum load current is 0.5A, for instance, a 0.5A inductor
may not survive a continuous 2A overload condition.
Also, the instantaneous application of input or release
from shutdown, at high input voltages, may cause
OUT MAX
OUT DIS
OUT MAX
IN
(
= 8V, V
)
(
2
OUT
V
1 5
1 5 0 23 1 27
( )( )
OUT
L f
.
.
= 5V and L = 3.3 H,
)
2 3 3 10
.
I
P
. •
V
.
OUT
5 8 5
6
2
A
L f V
1 25 10
.
V
IN
•
IN
V
OUT
6
sn1767 1767fas
8