LTC1142 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1142

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1142/LTC1142L/LTC1142HV

APPLICATIO S I FOR ATIO

dead-time, which could cost as much as 1% in efficiency

(although there are no other harmful effects if D1 and D2

are omitted). Therefore, D1 and D2 should be selected for

a forward voltage of less than 0.6V when conducting IMAX.

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle VOUT/ VIN. To

prevent large voltage transients, a low ESR input capaci-

tor sized for the maximum RMS current must be used. The

maximum RMS capacitor current is given by:

[ ( )]1/2

VOUT VIN − VOUT

CIN Required IRMS ≈ IMAX

VIN

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst case conditon is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on only 2000 hours

of life. This makes it advisable to further derate the

capacitor, or to choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

paralleled to meet size or height requirements in the

design. Always consult the manufacturer if there is any

question. An additional 0.1µF to 1µF ceramic capacitor is

also required on each VIN line (Pins 10 and 24) for high

frequency decoupling.

The selection of COUT is driven by the required Effective

Series Resistance (ESR). The ESR of COUT must be less

than twice the value of RSENSE for proper operation of the

LTC1142:

COUT Required ESR < 2RSENSE

Optimum efficiency is obtained by making the ESR equal

to RSENSE. As the ESR is increased up to 2RSENSE, the

efficiency degrades by less than 1%. If the ESR is greater

than 2RSENSE, the voltage ripple on the output capacitor

will prematurely trigger Burst Mode operation, resulting in

disruption of continuous mode and an efficiency hit which

can be several percent.

Manufacturers such as Nichicon and United Chemicon

should be considered for high performance capacitors.

The OS-CON semiconductor dielectric capacitor available

from Sanyo has the lowest ESR/size ratio of any aluminum

electrolytic at a somewhat higher price. Once the ESR

requirement for COUT has been met, the RMS current

rating generally far exceeds the IRIPPLE(P-P) requirement.

In surface mount applications multiple capacitors may

have to be parallel to meet the capacitance, ESR or RMS

current handling requirements of the application. Alumi-

num electrolytic and dry tantalum capacitors are both

available in surface mount configurations. In the case of

tantalum, it is critical that the capacitors are surge tested

for use in switching power supplies. An excellent choice

is the AVX TPS series of surface mount tantalums, avail-

able in case heights ranging from 2mm to 4mm. For

example, if 200µF/10V is called for in an application

requiring 3mm height, two AVX 100µF/10V (P/N TPSD

107K010) could be used. Consult the manufacturer for

other specific recommendations.

At low supply voltages, a minimum capacitance at COUT is

needed to prevent an abnormal low frequency operating

mode (see Figure 4). When COUT is made too small, the

output ripple at low frequencies will be large enough to trip

the voltage comparator. This causes Burst Mode opera-

tion to be activated when the LTC1142 would normally be

in continuous operation. The output remains in regulation

at all times.

1000

800

600

L = 50µH

RSENSE = 0.02Ω

L = 25µH

RSENSE = 0.02Ω

400

L = 50µH

200

RSENSE = 0.05Ω

0

0

1

2

3

4

5

VIN – VOUT VOLTAGE (V)

1142 F04

Figure 4. Minimum Value of COUT

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in DC (resistive) load

12

Share Link: