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LT3487

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 ampliﬁer

A1, and is simply an ampliﬁed version of the difference

between the feedback voltage and the reference voltage

of 1.23V. In this manner, the error ampliﬁer sets the cor-

rect peak current level to keep the output in regulation.

If the error ampliﬁer’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 ampliﬁer A3 and PWM comparator A4.

The reference voltage of this negative channel is ground.

Voltage clamps on VCPandVCN(notshown)enforcecurrent

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 VBAT voltage. The drive current for the output

disconnect PNP is returned to the VBAT pin. This allows

the pass transistor to turn off when the CAP voltage

falls to less than 1.2V above VBAT. The VBAT pin allows

applications in which the power (inductors L1 and L2)

and internal control circuitry (VIN 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 efﬁciency 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

0.18

580

Toko

www. tokoam.com

CDRH3D18-100

CDRH2D18HP-100

CDRH3D23-100

CDRH2D18/HP-150

CDRH3D18-150

CDRH3D23-150

0.205

0.245

0.117

0.345

0.301

0.191

900

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 sufﬁcient 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

Figure 2. Switching Waveforms

VSWP

20V/DIV

VSWN

20V/DIV

ISWN

100mA/DIV

200ns/DIV

3487 F02

VIN = 3.6V

VPOS = 15V, 25mA

VNEG = –8V, 50mA

ILI

100mA/DIV

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