LTC1612 Просмотр технического описания (PDF) - Linear Technology
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LTC1612 Datasheet PDF : 8 Pages
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OPERATIO
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 layouts.
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.
LT1612
consumption but also adds low frequency voltage ripple to
the output. Figure 3 shows switching waveforms for a 5V
to 3.3V converter running in Burst Mode operation. Output
voltage ripple is approximately 20mVP-P. If the MODE pin
is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.
VOUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
5µs/DIV
1612 F03
Figure 3. Output Voltage Ripple is 20mVP-P for
the Circuit of Figure 1
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
Burst Mode Operation Defeat
To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160µA. This reduces power
VOUT
200mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
10mA TO 310mA
0.1ms/DIV
1612 F04
Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating
VOUT
200mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
10mA TO 300mA
0.1ms/DIV
1612 F05
Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present
5
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