6
LT1612
sn1612 1612fs
Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and V
C
Pin Is Kept Low. Lead Length to Battery Should Be Kept Short
1
2
8
7
3
4
6
5
L1
C2
R1
C3
LT1612
V
OUT
V
IN
GND
SHDN
1612 F02
MODE
MULTIPLE
VIAs
C1
R2 R
C
C
C
OPERATIO
U
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 V
OUT
and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the V
C
pin either higher or lower.
Because switch current is proportional to the V
C
pin
voltage, this change causes the switch current to be
adjusted until V
OUT
is once again satisfied. Loop compen-
sation is taken care of by an RC network from the V
C
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 V
IN
. Current ramps up in the inductor linearly
at a rate of (V
IN
– V
OUT
)/L. Switch current is set by the V
C
pin voltage and when the voltage across R
SENSE
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 V
OUT
/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
If the load is light and V
C
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 C
IN
to
V
IN
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.
