Datasheet

LT3692A
16
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
can be set between 2A and 4.8A for each channel via a
resistor from the ILIM pin to ground. The ILIM pin is
driven by a 12µA current source. Setting resistor R
LIM
sets the voltage present at the ILIM pin which determines
the maximum switch current as illustrated in Figure 6. A
capacitor from the ILIM pin to ground, or a resistor divider
from the output, can be used to limit the peak current dur-
ing start-up. If a capacitor is used it must be discharged
before power
-up to ensure proper operation (see 3.3V and
2.5V Dual Step-Down Converter in T
ypical Applications).
Referring to Figure 6, as the peak current limit is reduced,
slope compensation further reduces the peak current with
increasing duty cycle.
When the ILIM pin is used to reduce the peak switch cur
-
rent, the equation for inductor choice becomes:
L =
50 • V
OUT
f • R
ILIM
for R
ILIM
> 24.9kΩ
where f is frequency in MHz, L in µH and R in kΩ.
When the LT3692A’s input supplies are operated at different
input voltages, an input capacitor sized for that channel
should be placed as close as possible to the respective
V
IN
pins.
A caution regarding the use of ceramic capacitors at the
input. A ceramic input capacitor can combine with stray
inductance to form a resonant tank circuit. If power is
applied quickly (for example by plugging the circuit into
a live power source) this tank can ring, doubling the input
voltage and damaging the LT3692A. The solution is to
either clamp the input voltage or dampen the tank circuit
by adding a lossy capacitor in parallel with the ceramic
capacitor. For details, see Application Note 88.
Output Capacitor Selection
Typically step-down regulators are easily compensated with
an output crossover frequency that is 1/10 of the switch
-
ing frequency. This means that the time that the output
capacitor must supply the output load during a transient
step is ~2
or 3 switching periods. With an allowable 1%
drop in output voltage during the step, a good starting
value for the output capacitor can be expressed by:
C
VOUT
=
Max Load Step
Frequency • 0.01• V
OUT
Example:
V
OUT
= 3.3V, Frequency = 1MHz, Max Load Step = 2A.
C
VOUT
=
2
1E6 • 0.01• 3.3V
= 60µF
The calculated value is only a suggested starting value.
Increase the value if transient response needs improvement
or reduce the capacitance if size is a priority. The output
capacitor filters the inductor current to generate an output
with low voltage ripple. It also stores energy in order to
satisfy transient loads and to stabilize the LT3692A’s control
loop. The switching frequency of the LT3692A determines
the value of output capacitance required. Also, the current
mode control loop doesn’t require the presence of output
capacitor series resistance (ESR). For these reasons, you
are free to use ceramic capacitors to achieve very low
output ripple and small circuit size.
Figure 6. Peak Switch Current vs ILIM Resistor
Input Capacitor Selection
Bypass the inputs of the LT3692A circuit with a 4.7µF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors.
ILIM PIN RESISTOR (kΩ)
0
1.0
PEAK SWITCH CURRENT (A)
1.5
2.5
3.0
3.5
4.5
10
50
70
3692a F06
2.0
4.0
40
90
100
20
30
60 80