LT1612 Просмотр технического описания (PDF) - Linear Technology

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LT1612

Synchronous, Step-Down 800kHz PWM DC/DC Converter

Linear Technology 

LT1612

U

OPERATIO

The LT1612 employs fixed frequency, current mode con-

trol. This type of control uses two feedback loops. The

main control loop sets output voltage and operates as

follows: A load step causes VOUT and the FB voltage to be

perturbed slightly. The error amplifier responds to this

change in FB by driving the VC pin either higher or lower.

Because switch current is proportional to the VC pin

voltage, this change causes the switch current to be

adjusted until VOUT is once again satisfied. Loop compen-

sation is taken care of by an RC network from the VC pin

to ground.

Inside this main loop is another that sets current limit on

a cycle-by-cycle basis. This loop utilizes current compara-

tor A2 to control peak current. The oscillator runs at

800kHz and issues a set pulse to the flip-flop at the

beginning of each cycle, turning the switch on. With the

switch now in the ON state the SW pin is effectively

connected to VIN. Current ramps up in the inductor linearly

at a rate of (VIN – VOUT)/L. Switch current is set by the VC

pin voltage and when the voltage across RSENSE trips the

current comparator, a reset pulse will be generated and the

switch will be turned off. Since the inductor is now loaded

up with current, the SW pin will fly low and trigger the

rectifier to turn on. Current will flow through the rectifier

decreasing at a rate of VOUT/L until the oscillator issues a

new set pulse, causing the cycle to repeat.

If the load is light and VC decreases below A3’s trip point,

the device will enter the Burst Mode operation region (the

MODE pin must be at ground or floating). In this state the

oscillator and all other circuitry except the reference and

comparator A3 are switched on and off at low frequency.

This mode of operation increases efficiency at light loads

but introduces low frequency voltage ripple at the output.

For continuous switching and no low frequency output

voltage ripple, pull the MODE pin high. This will disable

comparator A3 which forces the oscillator to run

continuously.

Layout Hints

The LT1612 switches current at high speed, mandating

careful attention to layout for proper performance. You

will not get advertised performance with careless layout.

Figure 2 shows recommended component placement for

a buck (step-down) converter. Follow this closely in your

PC layout. Note the direct path of the switching loops.

Input capacitor C1 must be placed close (< 5mm) to the IC

package. As little as 10mm of wire or PC trace from CIN to

VIN will cause problems such as inability to regulate or

oscillation.

The ground terminal of input capacitor C1 should tie close

to Pin 4 of the LT1612. Doing this reduces dI/dt in the

ground copper which keeps high frequency spikes to a

minimum. The DC/DC converter ground should tie to the

PC board ground plane at one place only, to avoid intro-

ducing dI/dt in the ground plane.

R1 R2 RC CC

1

8 SHDN

2

LT1612

7

MODE

3

6

VIN

C1 4

5

C3

C2

MULTIPLE

VIAs

GND

L1

1612 F02

VOUT

Figure 2. Recommended Component Placement. Traces

Carrying High Current are Direct. Trace Area at FB Pin and VC

Pin Is Kept Low. Lead Length to Battery Should Be Kept Short

6

sn1612 1612fs

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