LT1767EMS8-5#PBF Linear Technology, LT1767EMS8-5#PBF Datasheet - Page 7

• Manufacturer Part Number: LT1767EMS8-5#PBF
• Description: IC SW REG STEP-DOWN 5V 8-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1767EMS8-5#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 1.25MHz
• Voltage - Input: 3 ~ 25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Primary Input Voltage: 15V
• No. Of Outputs: 1
• Output Voltage: 5V
• Output Current: 1.5A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Supply Voltage Range: 3V To 25V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS
FB RESISTOR NETWORK
If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required, the
respective fixed option part, -1.8, -2.5, -3.3 or -5, should be
used. The FB pin is tied directly to the output; the necessary
resistive divider is already included on the part. For other
voltage outputs, the adjustable part should be used and an
external resistor divider added. The suggested resistor (R2)
from FB to ground is 10k. This reduces the contribution of
FB input bias current to output voltage to less than 0.25%.
The formula for the resistor (R1) from V
INPUT CAPACITOR
Step-down regulators draw current from the input supply in
pulses. The rise and fall times of these pulses are very fast.
The input capacitor is required to reduce the voltage ripple
this causes at the input of LT1767 and force the switching
current into a tight local loop, thereby minimizing EMI. The
RMS ripple current can be calculated from:
Higher value, lower cost ceramic capacitors are now avail-
able in smaller case sizes. These are ideal for input bypass-
ing since their high frequency capacitive nature removes
most ripple current rating and turn-on surge problems. At
higher switching frequency, the energy storage require-
ment of the input capacitor is reduced so values in the range
of 1 F to 4.7 F are suitable for most applications. Y5V or
similar type ceramics can be used since the absolute value
R
I
RIPPLE RMS
1
V
LT1767
C
1 2
R V
.
GND
2
AMPLIFIER
R
ERROR
OUT
2 0 25
Figure 2. Feedback Network
( .
I
OUT
U
1 2
+
–
.
V
A
OUT
INFORMATION
)
1.2V
U
V
IN
V
SW
FB
V
W
OUT
R1
R2
10k
OUT
/
+
V
IN
1767 F02
to FB is:
2
OUTPUT
U
LT1767-2.5/LT1767-3.3/LT1767-5
of capacitance is less important and has no significant
effect on loop stability. If operation is required close to the
minimum input required by the output of the LT1767, a
larger value may be required. This is to prevent excessive
ripple causing dips below the minimum operating voltage,
resulting in erratic operation.
If tantalum capacitors are used, values in the 22 F to 470 F
range are generally needed to minimize ESR and meet ripple
current and surge ratings. Care should be taken to ensure
the ripple and surge ratings are not exceeded. The AVX TPS
and Kemet T495 series are surge rated. AVX recommends
derating capacitor operating voltage by 2:1 for high surge
applications.
OUTPUT CAPACITOR
Unlike the input capacitor, RMS ripple current in the output
capacitor is normally low enough that ripple current rating
is not an issue. The current waveform is triangular, with an
RMS value given by:
The LT1767 will operate with both ceramic and tantalum
output capacitors. Ceramic capacitors are generally chosen
for their small size, very low ESR (effective series resis-
tance), and good high frequency operation, reducing out-
put ripple voltage. Their low ESR removes a useful zero in
the loop frequency response, common to tantalum capaci-
tors. To compensate for this, the V
pole frequency must typically be reduced by a factor of 10.
Typical ceramic output capacitors are in the 1 F to 10 F
range. Since the absolute value of capacitance defines the
pole frequency of the output stage, an X7R or X5R type
ceramic, which have good temperature stability, is recom-
mended.
Tantalum capacitors are usually chosen for their bulk
capacitance properties, useful in high transient load appli-
cations. ESR rather than capacitive value defines output
ripple at 1.25MHz. Typical LT1767 applications require a
tantalum capacitor with less than 0.3
500 F, see Table 2.
I
RIPPLE RMS
0 29
.
LT1767/LT1767-1.8/
V
OUT
L f V
V
IN
IN
V
C
OUT
loop compensation
ESR at 22 F to
sn1767 1767fas
7