Datasheet

LT3686
15
3686fc
Figure 7 shows the transient response of the LT3686 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 0.25A to 1A and back to 0.25A,
and the oscilloscope traces show the output voltage. The
upper photo shows the recommended value. The second
photo shows the improved response (less voltage drop)
resulting from a larger output capacitor and a phase lead
capacitor. The last photo shows the response to a high
performance electrolytic capacitor. Transient performance
is improved due to the large output capacitance.
ApplicAtions inForMAtion
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost diode are used to
generate a boost voltage that is higher than the input
voltage. In most cases a 0.22μF capacitor will work well.
Figure 8 shows two ways to arrange the boost circuit. The
BOOST pin must be at least 2.2V above the SW pin for
best efficiency. For outputs of 3V and above, the standard
circuit (Figure 8a) is best. For outputs less than 3V and
above 2.5V, place a discrete Schottky diode (such as the
Figure 7. Transient Load Response of the LT3686 with Different Output Capacitors as the Load Current Is Stepped from 0.25A to 1A.
V
IN
= 12V, V
OUT
= 3.3V, L = 6.8µH , Frequency = 2MHz
32.4k
10k
FB
V
OUT
22µF
32.4k
10k
FB
V
OUT
47pF
22µF
×2
32.4k
10k
FB
V
OUT
100µF
SANYO
4TPB100M
+
3686 F07a
3686 F07d
3686 F07g
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
3686 F07b
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
MODE < 0.4V MODE > 0.8V
3686 F07c
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
3686 F07e
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
3686 F07h
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
3686 F07i
V
OUT
50mV/DIV
AC
I
L
500mA/DIV
20µs/DIV
3686 F07f