LT1949-1 Datasheet, PDF (5/8 Pages) Linear Technology – 1.1MHz, 1A Switch PWM DC/DC Converter

LT1949-1
BLOCK DIAGRA
1.24V
REFERENCE
FB
2
V_OUT
R1
(EXTERNAL)
FB
R2
(EXTERNAL)
+
g_m
–
ERROR
AMPLIFIER
RAMP
GENERATOR
1.1MHz
OSCILLATOR
LBI
V_C
7+
LBO
1
8
+
BIAS –
200mV
ENABLE
A1
COMPARATOR
–
A4
SHUTDOWN
SHDN
3
–
FF
+
Σ
+
R
Q
S
+
A2
COMPARATOR
SW
5
DRIVER
Q3
A=2
0.06Ω
4
1949-1 BD
GND
Figure 3. LT1949-1 Block Diagram
U
OPERATIO
The LT1949-1 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 V_C
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, V_OUT (and FB) will drop slightly and the error
amplifier will drive V_C to a higher voltage, causing current
in the switch to increase. In this way, the error amplifier
drives the V_C pin to the voltage necessary to satisfy the
load. Frequency compensation is provided by an external
series RC network connected between the V_C pin and
ground.
Layout Hints
The LT1949-1 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 C_IN to
V_IN 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 LT1949-1. 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