LTC3388-1_15 Datasheet, PDF (17/20 Pages) Linear Technology – 20V High Efficiency Nanopower Step-Down Regulator

LTC3388-1/LTC3388-3
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.
2.7V TO 4.2V
+
Li-ION
10μF
6V
V_IN
STBY
V_IN2
PGOOD
EN ^LTC3388-1
D1
SW
CAP
V_OUT
D0
GND
22μH
STBY
EN
T_X
1M
MICROPROCESSOR
1.8V
CORE _GND
47μF
6V
338813 F07a
V_OUT
20mV/DIV
AC-COUPLED
LOAD
CURRENT
25mA/DIV
0mA
250μs/DIV
338813 F07b
V_IN = 5.5V
L = 22μH, C_OUT = 100μF
LOAD STEP BETWEEN 5mA AND 50mA
Figure 7. 1.8V Step-Down Converter Powering a Microprocessor
with a Wireless Transmitter and 45mA Load Step Response
MIDE V21BL
PZ1
PZ2
1μF
6V
10μF
25V
V_IN
PGOOD
LTC3588-1
CAP
SW
V_IN2
V_OUT
47μH
D1
4.7μF
6V
D0
GND
1M
3.3V
10μF
6V
EN
PGOOD
V_IN2 ^LTC3388-1
V_IN
SW
CAP
V_OUT
D1
D0
GND ^STBY
22μH
1.2V
47μF
6V
338813 F07
Figure 8. Piezoelectric Energy Harvester and 1.2V Secondary Rail
338813f
17