
LT3587
11
3587fc
Figure 3. Boost3 Congured as a Voltage
Regulator and as a Current Regulator
capacitance will slow down the switching waveform, which
can cause signicant switching losses at 1MHz switch-
ing frequency. Some recommended Schottky diodes are
listed in Table 2.
Table 2. Recommended Schottky Diodes
PART
NUMBER
FORWARD
CURRENT
(mA)
FORWARD
VOLTAGE
DROP (V)
DIODE
CAPACI-
TANCE
(pF at 10V) MANUFACTURER
RSX051VA-30
1000
0.35
30
ROHM
www.rohm.com
PMEG401OCEJ
500
0.49
25
NXP/Phillips
www.nxp.com
PMEG2005EB
500
0.43
8
IR05H40CSPTR
500
0.48
39
Vishay
www.vishay.com
B0540WS
500
0.48
20
Diodes Inc.
www.diodes.com
ZLLS400
520
0.53
17
Zetex
www.zetex.com
Smaller Footprint Inverter Topology
In certain applications with higher tolerance of current ripple
at the output of the inverter, the inductor L3 can be replaced
with a Schottky diode. Since the Schottky diode footprint
is usually smaller than the inductor footprint, this alternate
topology is recommended if a smaller overall solution is a
must. Note that this topology is only viable if the absolute
value of the inverter output is greater than VIN.
This Schottky diode is congured with the anode connected
to the output of the inverter and the cathode to the output
end of the ying capacitor C2 as shown in Figure 2.
APPLICATIONS INFORMATION
The same constraints as the other Schottky diodes ap-
ply for selecting D3. Therefore, the same recommended
Schottky diodes in Table 2 can be used for D3.
Boost3 Overcurrent and Overvoltage Protection
As briey discussed in the Operation section, the regula-
tion loop of Boost3 uses the maximum of the two voltages
at VFB3 and IFB3 as feedback information to set the peak
current of its power switch Q3. This allows for the Boost3
loop to be congured as either a boost voltage regulator
or a boost current regulator (Figure 3). Furthermore, this
architecture also allows for a programmable current limit on
voltage regulation or voltage limit on current regulation.
Figure 2. Inverter Congured with a Schottky
Diode in Place of the Output Inductor
3587 F02
SW2
FB2
LT3587
RFB1
1M
VVIN
2.5V TO 4.5V
C2
2.2μF
L2
15μH
D3
INVERTER
OUTPUT
–8V, 100mA
DS2
C7
22μF
3587 F03
LT3587
BOOST3
VOLTAGE
REGULATOR
VIN
VVIN
SW3
EN/SS3
IFB3
CAP3
VOUT3
VFB3
RVFB3
VOLTAGE
REGULATION
FEEDBACK
RESISTOR
RIFB3
OPTIONALPROGRAMMABLE
CURRENT LIMIT RESISTOR
LT3587
BOOST3
CURRENT
REGULATOR
VIN
VVIN
SW3
EN/SS3
IFB3
CAP3
VOUT3
VFB3
RVFB3
OPTIONAL
PROGRAMMABLE
VOLTAGE LIMIT
RESISTOR
RIFB3
CURRENT REGULATION
FEEDBACK RESISTOR
When congured as a boost voltage regulator, a feedback
resistor from the output pin VOUT3 to the VFB3 pin sets the
voltage level at VOUT3 at a xed level. In this case, the IFB3
pin can either be grounded if no current limiting is desired
or connected to ground with a resistor such that:
ILIMIT = 200 (0.8V/RIFB3)
where ILIMIT is the desired output current limit value. Recall
that the pull-up current on the IFB3 pin is controlled to be
typically 1/200 of the output load current at the VOUT3
pin. In this case, when the load current is less than ILIMIT,
the Boost3 loop regulates the voltage at the VFB3 pin to
0.8V. When there is an increase in load current beyond
ILIMIT, the voltage at VFB3 starts to drop and the voltage
at IFB3 rises above 0.8V. The Boost3 loop then regulates
the voltage at the IFB3 pin to 0.8V, limiting the output