LT1578IS8 Linear Technology, LT1578IS8 Datasheet - Page 18

• Manufacturer Part Number: LT1578IS8
• Description: IC BUCK SYNC ADJ 1.5A 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1578IS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.21V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 4 ~ 15 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT1578/LT1578-2.5
APPLICATIONS
because at 200kHz, any value above 15 F is essentially
resistive. RMS ripple current rating is the critical param-
eter. Actual RMS current can be calculated from:
The term inside the radical has a maximum value of 0.5
when input voltage is twice output, and stays near 0.5 for
a relatively wide range of input voltages. It is common
practice therefore to simply use the worst-case value and
assume that RMS ripple current is one half of load current.
At maximum output current of 1.5A for the LT1578, the
input bypass capacitor should be rated at 0.75A ripple
current. Note however, that there are many secondary
considerations in choosing the final ripple current rating.
These include ambient temperature, average versus peak
load current, equipment operating schedule, and required
product lifetime. For more details, see Application Notes
19 and 46, and Design Note 95.
Input Capacitor Type
Some caution must be used when selecting the type of
capacitor used at the input to regulators. Aluminum
electrolytics are lowest cost, but are physically large to
achieve adequate ripple current rating, and size con-
straints (especially height) may preclude their use.
Ceramic capacitors are now available in larger values, and
their high ripple current and voltage rating make them
ideal for input bypassing. Cost is fairly high and footprint
may also be somewhat large. Solid tantalum capacitors
would be a good choice, except that they have a history of
occasional spectacular failures when they are subjected to
large current surges during power-up. The capacitors can
short and then burn with a brilliant white light and lots of
nasty smoke. This phenomenon occurs in only a small
percentage of units, but it has led some OEMs to forbid
their use in high surge applications. The input bypass
capacitors of regulators can see these high surges when
a battery or high capacitance source is connected. Several
manufacturers have developed a line of solid tantalum
capacitors specially tested for surge capability (AVX TPS
18
I
RIPPLE RMS
I
OUT
U
INFORMATION
V
U
OUT
V
IN
W
V
OUT
V
IN
U
2
series for instance, see Table 3), but even these units may
fail if the input voltage surge approaches the maximum
voltage rating of the capacitor. AVX recommends derating
capacitor voltage by 2:1 for high surge applications. The
highest voltage rating is 50V, so 25V may be a practical
input voltage upper limit when using solid tantalum ca-
pacitors for input bypassing.
Larger capacitors may be necessary when the input volt-
age is very close to the minimum specified on the data
sheet. Small voltage dips during switch on time are not
normally a problem, but at very low input voltage they may
cause erratic operation because the input voltage drops
below the minimum specification. Problems can also
occur if the input-to-output voltage differential is near
minimum. The amplitude of these dips is normally a
function of capacitor ESR and ESL because the capacitive
reactance is small compared to these terms. ESR tends to
be the dominate term and is inversely related to physical
capacitor size within a given capacitor type.
SYNCHRONIZING
The SYNC pin is used to synchronize the internal oscillator
to an external signal. The SYNC input must pass from a
logic level low, through the maximum synchronization
threshold with a duty cycle between 10% and 90%. The
input can be driven directly from a logic level output. The
synchronizing range is equal to initial operating frequency
up to 400kHz. This means that minimum practical sync
frequency is equal to the worst-case high self-oscillating
frequency (250kHz), not the typical operating frequency of
200kHz. Caution should be used when synchronizing
above 280kHz because at higher sync frequencies the
amplitude of the internal slope compensation used to
prevent subharmonic switching is reduced. This type of
subharmonic switching only occurs at input voltages less
than twice output voltage. Higher inductor values will tend
to eliminate this problem. See Frequency Compensation
section for a discussion of an entirely different cause of
subharmonic switching before assuming that the cause is
insufficient slope compensation. Application Note 19 has
more details on the theory of slope compensation.