LTC1627 Просмотр технического описания (PDF) - Linear Technology

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LTC1627

Monolithic Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1627

APPLICATIONS INFORMATION

The selection of COUT is driven by the required effective series

resistance (ESR). Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering. The output

ripple ∆VOUT is determined by:

∆VOUT

≅



∆ILESR

+

1

4fCOUT





where f = operating frequency, COUT = output capacitance

and ∆IL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ∆IL increases

with input voltage. For the LTC1627, the general rule for

proper operation is:

COUT required ESR < 0.25Ω

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole 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 paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic 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 tantalum, available in case

heights ranging from 2mm to 4mm. Other capacitor types

include Sanyo POSCAP, KEMET T510 and T495 series,

Nichicon PL series and Sprague 593D and 595D series.

Consult the manufacturer for other specific recommenda-

tions.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

VOUT

=

0.8V



1

+

R2 

R1

(2)

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 4.

0.8V ≤ VOUT ≤ 8.5V

R2

VFB

LTC1627

R1

GND

1627 F04

Figure 4. Setting the LTC1627 Output Voltage

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft-start function and a means to shut down the LTC1627.

Soft-start reduces surge currents from VIN by gradually

increasing the internal current limit. Power supply

sequencing can also be accomplished using this pin.

An internal 2.25µA current source charges up an external

capacitor CSS. When the voltage on RUN/SS reaches 0.7V

the LTC1627 begins operating. As the voltage on RUN/SS

continues to ramp from 0.7V to 1.8V, the internal current

limit is also ramped at a proportional linear rate. The

current limit begins at 25mA (at VRUN/SS ≤ 0.7V) and ends

at the Figure 3 value (VRUN/SS ≈ 1.8V). The output current

thus ramps up slowly, charging the output capacitor. If

RUN/SS has been pulled all the way to ground, there will

be a delay before the current starts increasing and is given

by:

tDELAY

=

0.7CSS

2.25µA

Pulling the RUN/SS pin below 0.4V puts the LTC1627 into

a low quiescent current shutdown (IQ < 15µA). This pin can

be driven directly from logic as shown in Figure 5. Diode

D1 in Figure 5 reduces the start delay but allows CSS to

ramp up slowly providing the soft-start function. This

diode can be deleted if soft-start is not needed.

3.3V OR 5V

RUN/SS

D1

RUN/SS

CSS

CSS

1627 F05

Figure 5. RUN/SS Pin Interfacing

9

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