LT1176CS Просмотр технического описания (PDF) - Linear Technology

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LT1176CS

Step-Down Switching Regulator

Linear Technology 

LT1176/LT1176-5

ELECTRICAL CHARACTERISTICS TJ = 25°C, VIN = 25V, unless otherwise noted

PARAMETER

Feedback Pin Bias Current (LT1176)

Reference Voltage (LT1176)

Reference Voltage Tolerance (LT1176)

Sense Voltage (LT1176-5)

Sense Voltage Tolerance (LT1176-5)

Sense Pin Divider Resistance (LT1176-5)

Output Voltage Line Regulation

VC Voltage at 0% Duty Cycle

Multiplier Reference Voltage

Shutdown Pin Current

VSHDN ≤ VTHRESHOLD (≅ 2.5V)

Shutdown Thresholds

Status Window

Status High Level

Status Low Level

Status Delay Time

Status Minimum Width

CONDITIONS

VFB = VREF

VC = 2V

VREF (Nominal) = 2.21V

All Conditions of Input Voltage, Output

Voltage, Temperature and Load Current

VC = 2V

VOUT (Nominal) = 5V

All Conditions of Input Voltage,

Temperature and Load Current

8V ≤ VIN ≤ VMAX (Note 7)

Over Temperature

VSHDN = 5Vq

q

Switch Duty Cycle = 0

Fully Shut Down

As a Percent of Output Voltage

ISTATUS = 10µA Sourcing

ISTATUS = 1.6mA Sinking

MIN TYP MAX

q

0.5

2

q 2.155 2.21 2.265

±0.5 ±1.5

q

±1.0 ±2.5

q 4.85

5

5.15

± 0.5

±2

q

± 1.0

±3

3

5

8

q

0.005 0.02

1.5

q

– 4.0

24

5

10

20

µA

50

µA

q 2.2

q 0.1

2.45 2.7

0.30 0.5

±4

±5

±6

q 3.5

4.5

5.0

q

0.25 0.4

9

30

UNITS

µA

V

%

%

V

%

%

kΩ

%/ V

V

mV/°C

V

V

V

%

V

V

µs

µs

The q denotes specifications which apply over the full operating tempera-

ture range.

Note 1: To calculate maximum switch “ON” voltage at current between low

and high conditions, a linear interpolation may be used.

Note 2: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low

clamp level and the switch duty cycle to zero. This approximates the zero

load condition where duty cycle approaches zero. The LT1176-5 has

VSENSE = 5.5V.

Note 3: Total voltage from VIN pin to ground pin must be ≥8V after start-

up for proper regulation.

Note 4: Switch frequency is internally scaled down when the feedback pin

voltage is less than 1.3V to avoid extremely short switch-on times. During

testing, VFB or VSENSE is adjusted to give a minimum switch-on time of 1µs.

Note 5: ILM = (RLIM – 1k)/ 7.65k

Note 6: Switch to input voltage limitation must also be observed.

Note 7: VMAX = 35V

Note 8: Does not include switch leakage.

Note 9: Error amplifier voltage gain and transconductance are specified

relative to the internal feedback node. To calculate gain and transconduc-

tance from the sense pin (Output) to the VC pin on the LT1176-5, multiply

by 0.44.

Application Hints

Although the LT1176 has a peak switch rating of 1.2A and a maximum duty

cycle of 85%, it must be used cautiously in applications which require high

switch current and high duty cycle simultaneously, to avoid excessive chip

temperature. Thermal resistance is 90°C/W for the 8-pin DIP package and

50°C/W for the 20-pin SO. This limits continuous chip power dissipation to

the 0.5W to 1W range. These numbers assume typical mounting techniques.

Extra or thick copper connected to the leads can reduce thermal resistance.

Bonding the package to the board or using a clip style heatsink can also help.

The following formulas will give chip power dissipation and peak switch

current for the standard buck converter. Note that surges less than 30

seconds do not need to be considered from a thermal standpoint, but for

proper regulation, they must not result in peak switch currents exceeding the

1.2A limit.

Power = ILOAD (VOUT/VIN) + VIN [7mA + 3mA (VOUT/VIN) + 0.012 (ILOAD)]

IPEAK = ILOAD(PEAK) + [VOUT (VIN – VOUT)]/2E5(VIN)(L)

Example: VIN = 15V, VOUT = 5V, ILOAD = 0.5A Continuous, 0.8A Peak,

L = 100µH

Power (ILOAD = 0.5A) = 0.38W

IPEAK (ILOAD = 0.8A) = 0.97A

Where component size or height is critical, we suggest using solid

tantalum capacitors (singly or in parallel), but be sure to use units rated

for switching applications. Coiltronics is a good source for low profile

surface mount inductors and AVX makes high quality surface mount

tantalum capacitors. For further help, use Application Notes 19 and 44,

LTC’s SwitcherCAD computer design program, and our knowledgeable

application department.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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