Datasheet
LTC3388-1/LTC3388-3
17
338813f
APPLICATIONS INFORMATION
Interfacing with a Microprocessor
The PGOOD, STBY, and EN pins can be useful when power-
ing a microprocessor from the LTC3388-1/LTC3388-3.
The PGOOD signal can be used to enable a sleeping micro-
processor or other circuitry when V
OUT
reaches regulation,
as shown in Figure 7. While active, a microprocessor may
draw a small load when operating sensors, and then draw a
large load to transmit data. Figure 7 shows the LTC3388-1/
LTC3388-3 responding smoothly to such a load step.
The microprocessor or other circuitry may require a quiet
supply for performing some functions. The STBY pin allows
the microprocessor to place the LTC3388-1/LTC3388-3 into
standby mode where the buck converter is inactive. Any
ripple in the output voltage of the LTC3388-1/LTC3388-3
will cease and the output capacitor will support the load of
the microprocessor and other circuitry. While in standby
the output voltage will decrease as it’s loaded. The output
capacitor should be sized to minimize the decline.
The EN pin can be used to activate the LTC3388-1/LTC3388-3.
For instance, in Figure 8 the LTC3388-1 is enabled by the
PGOOD output of the LTC3588-1, a piezoelectric energy
harvesting power supply, to create a 1.2V rail. The quies-
cent current that the LTC3388-1 draws will appear at the
input of the LTC3588-1, reduced by the conversion ratio
of the LTC3588-1 buck converter. Because the LTC3388-1
is driven by a 3.3V supply no capacitors are needed for
the internal V
IN2
and CAP rails.
Figure 7. 1.8V Step-Down Converter Powering a Microprocessor
with a Wireless Transmitter and 45mA Load Step Response
Figure 8. Piezoelectric Energy Harvester and 1.2V Secondary Rail
338813 F07
PZ1
V
IN
CAP
V
IN2
D1
D0
PZ2
PGOOD
SW
V
OUT
LTC3588-1
MIDE V21BL
GND
10µF
25V
10µF
6V
1M
47µH
1µF
6V
4.7µF
6V
EN
V
IN2
V
IN
CAP
D1
D0
PGOOD
SW
V
OUT
STBY
LTC3388-1
GND
47µF
6V
1.2V
3.3V
22µH
250µs/DIV
V
OUT
20mV/DIV
AC-COUPLED
V
IN
= 5.5V
L = 22µH, C
OUT
= 100µF
LOAD STEP BETWEEN 5mA AND 50mA
LOAD
CURRENT
25mA/DIV
0mA
338813 F07b
338813 F07a
V
IN
V
IN2
EN
D1
CAP
D0
STBY
PGOOD
SW
V
OUT
STBY
EN
CORE
T
X
LTC3388-1
GND
10µF
6V
Li-ION
2.7V TO 4.2V
+
47µF
6V
1M
22µH
1.8V
MICROPROCESSOR
GND