lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 15

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Figure 7 shows the transient response of the LT3686A 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.
FB
Figure 7. Transient Load Response of the LT3686A with Different Output Capacitors as the Load Current Is Stepped from 0.25A to
1A. V
32.4k
10k
IN
FB
FB
= 12V, V
32.4k
10k
32.4k
10k
47pF
OUT
+
= 3.3V, L = 6.8μH , Frequency = 2MHz
3686A F07a
22μF
×2
3686A F07d
3686A F07g
22μF
V
100μF
SANYO
4TPB100M
V
V
OUT
OUT
OUT
500mA/DIV
500mA/DIV
500mA/DIV
50mV/DIV
50mV/DIV
50mV/DIV
V
V
V
OUT
OUT
OUT
AC
AC
AC
I
I
I
L
L
L
SYNC/MODE < 0.4V
20μs/DIV
20μs/DIV
20μs/DIV
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 effi ciency. 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
3686A F07b
3686A F07h
3686A F07e
500mA/DIV
500mA/DIV
500mA/DIV
50mV/DIV
50mV/DIV
50mV/DIV
V
V
V
OUT
OUT
OUT
AC
AC
AC
I
I
I
L
L
L
SYNC/MODE > 0.8V
20μs/DIV
20μs/DIV
20μs/DIV
LT3686A
15
3686A F07c
3686A F07f
3686A F07i
3686af