LT1949 Ver la hoja de datos (PDF) - Linear Technology
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LT1949 Datasheet PDF : 8 Pages
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BLOCK DIAGRA
LT1949
1.24V
REFERENCE
FB
2
VOUT
R1
(EXTERNAL)
FB
R2
(EXTERNAL)
+
gm
–
ERROR
AMPLIFIER
RAMP
GENERATOR
600kHz
OSCILLATOR
LBI
VC
7+
LBO
1
8
–
A4
200mV
+
ENABLE
SHDN
BIAS –
A1
COMPARATOR
SHUTDOWN
3
SW
–
FF
+
Σ
+
R
Q
S
+
A2
COMPARATOR
5
DRIVER
Q3
A=2
0.06Ω
4
GND 1949 BD
Figure 3. LT1949 Block Diagram
U
OPERATIO
The LT1949 is a current mode, fixed frequency step-up
DC/DC converter with an internal 1A NPN power transis-
tor. Operation can best be understood by referring to the
Block Diagram.
At the beginning of each oscillator cycle, the flip-flop is set
and the switch is turned on. Current in the switch ramps
up until the voltage at A2’s positive input reaches the VC
pin voltage, causing A2’s output to change state and the
switch to be turned off. The signal at A2’s positive input is
a summation of a signal representing switch current and
a ramp generator (introduced to avoid subharmonic oscil-
lations at duty factors greater than 50%). If the load
increases, VOUT (and FB) will drop slightly and the error
amplifier will drive VC to a higher voltage, causing current
in the switch to increase. In this way, the error amplifier
drives the VC pin to the voltage necessary to satisfy the
load. Frequency compensation is provided by an external
series RC network connected between the VC pin and
ground.
Layout Hints
The LT1949 switches current at high speed, mandating
careful attention to layout for proper performance. You
will not get advertised performance with careless layouts.
Figure 4 shows recommended component placement for
a boost (step-up) 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 output capacitor C2 should tie
close to Pin 4 of the LT1949. 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.
5
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