LT3487
8
3487f
Operation
The LT3487 uses a constant frequency, current mode con-
trol scheme to provide excellent line and load regulation.
Operation can be best understood by referring to the Block
Diagram in Figure 1. At the start of each oscillator cycle,
the SR latch X1 is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the
SR latch X1 is reset, turning off the power switch Q1. The
level at the negative input of A2 is set by the error amplifi er
A1, and is simply an amplifi ed version of the difference
between the feedback voltage and the reference voltage
of 1.23V. In this manner, the error amplifi er sets the cor-
rect peak current level to keep the output in regulation.
If the error amplifi er’s output increases, more current is
delivered to the output; if it decreases, less current is
delivered. The second channel is an inverting converter.
The basic operation is the same as the positive channel.
The SR latch X2 is also set at the start of each oscillator
cycle. The power switch Q2 is turned on at the same time
as Q1. Q2 turns off based on its own feedback loop, which
consists of error amplifi er A3 and PWM comparator A4.
The reference voltage of this negative channel is ground.
Voltage clamps on V
CP
and V
CN
(not shown) enforce current
limit. Switching waveforms with typical load conditions
are shown in Figure 2.
The PNP Q3 is used as an output disconnect pass transistor.
Q3 disconnects the load from the input during shutdown.
The anti-sat driver keeps Q3 at the edge of saturation as
long as CAP is typically 1.2V and worst-case 1.6V (cold)
above the V
BAT
voltage. The drive current for the output
disconnect PNP is returned to the V
BAT
pin. This allows
the pass transistor to turn off when the CAP voltage
falls to less than 1.2V above V
BAT
. The V
BAT
pin allows
applications in which the power (inductors L1 and L2)
and internal control circuitry (V
IN
pin) are powered from
different sources.
Inductor Selection
A 10μH inductor is recommended for the LT3487 boost
channel. The inverting channel can use uncoupled 15μH
inductors, or coupled 10μH inductors. Small size and
high effi ciency are the major concerns for most LT3487
applications. Inductors with low core losses and small
DCR (copper wire resistance) at 2MHz are good choices
for LT3487 applications. The inductor DCR should be on
the order of half of the switch on-resistance for its chan-
nel. Some inductors in this category with small size are
listed in Table 1.
Table 1. Recommended Inductors
PART NUMBER
INDUCTANCE
(μH)
DCR
(Ω)
CURRENT
RATING
(mA) MANUFACTURER
DB318C-A997AS-
100M
10 0.18 580 Toko
www. tokoam.com
CDRH3D18-100
CDRH2D18HP-100
CDRH3D23-100
CDRH2D18/HP-150
CDRH3D18-150
CDRH3D23-150
10
10
10
15
15
15
0.205
0.245
0.117
0.345
0.301
0.191
900
850
850
700
750
700
Sumida
www.sumida.com
Capacitor Selection
The small size of ceramic capacitors makes them suitable
for LT3487 applications. X5R and X7R types of ceramic
capacitors are recommended because they retain their
capacitance over wider voltage and temperature ranges
than other types such as Y5V or Z5U. A 1μF input capaci-
tor is suffi cient for most LT3487 applications. The output
capacitors required for stability depend on the application.
For the typical Li-Ion to +15V, –8V application, the positive
channel requires a 4.7μF output capacitor and the negative
channel requires at least 10μF of capacitance.
APPLICATIO S I FOR ATIO
WUU
U
Figure 2. Switching Waveforms
V
SWP
20V/DIV
V
SWN
20V/DIV
I
SWN
100mA/DIV
200ns/DIV
3487 F02
V
IN
= 3.6V
V
POS
= 15V, 25mA
V
NEG
= –8V, 50mA
I
LI
100mA/DIV
