LINEAR TECHNOLOGY LT3587 Technical data

L DESIGN IDEAS

BATTERY VOLTAGE (V)

2.7

500

600

700

3.9

400

300

3.0 3.3 3.6 4.2

200

100

0

CHARGE CURRENT (mA)

V

BUS

= 5V

R

PROG

= 1k

R

CLPROG

= 3k

5x USB SETTING,
BATTERY CHARGER SET FOR 1A

Complete Power Solution for Digital
Cameras and Other Complex
Compact Portable Applications

Introduction

Digital cameras, portable GPS systems,
MP3 players and other feature-rich
mobile devices have complicated
power requirements. In these complex
devices, the flow of power must be
carefully managed between a num­ber of specialized sources and loads,
including charging/discharging the
battery, current-limited USB power
and a set of multivoltage power sup­ply rails, including negative rails for
CCDs or LCDs. The supply rails must
be sequenced and tracked and faults
must be handled cleanly and com­municated to a microcontroller.

When these requirements are
added together, the task of squeezing
an efficient and robust power system
into a handheld device can seem near
impossible. Linear Technology solves
this problem with a family of devices
called PMICs (Power Management In­tegrated Circuits) that greatly simplify
the design of complex rechargeable
battery power systems.

Some Linear Technology PMICs
use a switching PowerPath controller
topology with the unique Bat-Track
feature, which allows charge currents
above the USB limit (see Figure 1) for
faster battery charging. The power
solution for digital cameras presented

34

34

Figure 1. Battery charge current
vs battery voltage

The LTC3586 implements

Linear Technology’s unique

Bat-Track™ technology,

which can use more power

from a USB source than

traditional linear chargers,

resulting in faster charging.

here takes advantage of this and other
powerful PMIC features.

Complete Digital Camera
Power System

Figure 2 shows a complete digital
camera power solution using the
LTC3586 PMIC as the power traffic
control center. Its 4mm × 6mm QFN
package includes a USB PowerPath
manager, a battery charger, plus a
boost DC/DC converter, a buck-boost
and two buck converters. The LT3587
in a 3mm × 3mm package is used to
drive a CCD and an LED backlight
for an LCD screen with a high voltage
monolithic inverter and dual boost
converter.

Switching PowerPath
Controller Maximizes
Available Power

The LTC3586 implements Linear
Technology’s unique Bat-T rack™
technology, which maximizes the use
of available power from a USB source
for either providing current to the
load or charging the battery at rates
greater than achievable from linear
chargers.

The switching PowerPath control­ler maintains accurate control of the
average input current for USB ap­plications. The average level of input
current is controlled by the state
of two digital inputs and can be set

by Brian Shaffer

to 100mA, 500mA, 1A or suspend
(500µA). The switching PowerPath
controller is highly efficient, which
results in battery charge currents of
well over 600mA from a 500mA USB
source (Figure 1).

The battery charging efficiency is
between 85% and 90% for the entire
battery voltage range. In contrast,
the efficiency of a traditional linear
charger falls as low as 57%, generating
the losses as heat. See Figure 3 for a
graph of the battery charger efficiency
as a function of battery voltage.

Instant-On Operation

The LTC3586 also features instant-on
operation, which allows the camera to
function immediately when external
power is applied even if the battery
voltage is below the system cutoff
voltage. This is achieved by generating
a separate voltage rail, V
is decoupled from the battery volt­age when the battery is below 3.3V.
When external power is applied, the
PowerPath controller prioritizes load
current over battery charge current
and regulates V

to 3.6V, enabling

OUT

the system to operate immediately
upon the application of external power.
The instant-on feature is important in
camera applications because impor ­tant moments do not wait for batteries
to charge.

Fault Handling

The FAULT signals on both of these
devices are designed to work together
for seamless fault handling. By making
the fault signals both an input and
an output, the two chips can com­municate fault events to each other.
If either of the devices has a fault then
all the outputs turn off, protecting
the system and battery from damage.
The enable lines and the fault signal

Linear Technology Magazine • September 2008

, which

OUT

+

MP1

C2
22µF

Li-Ion

510Ω

324k

121k

105k

RED

3.3V
1A

1.8V

400mA

MICROPROCESSOR

1.6V

400mA

5V

800mA

15V

50mA

BACKLIGHT LEDs

–8V

100mA

L1

3.3µH

L2

2.2µH

SW

V

BUS

T

NTC

PROG

CLPROG

100k

LTC3586

V

OUT3

V

C3

3.3V, 20mA

FB3

SWCD3

SWAB3

LDO3V3

I

LIM

MODE

FAULT

4

EN

V

OUT

BAT

GND

CHRG

GATE

2.2µF

330pF

33pF

10pF

15k

1µF

1µF

22µF

V

IN3

10pF1.02M

C1, C2: TDK C2012X5R0J226M
L1: COILCRAFT LPS4018-332LM
L2, L5: TOKO 1098AS-2R2M
L3, L4: TOKO 1098AS-4R7M
MP1: SILICONIX Si2333

806k

10k

L3

4.7µH

SW2

FB2

I/O/MEMORY

CORE

10µF

V

IN2

10pF

10pF

806k

806k

L4

4.7µH

L5

2.2µH

SW1

FB1

10µF

88.7k

16.9k

V

IN1

SW4

V

OUT4

FB4

V

IN4

SYSTEM RAIL/

I/O

AUDIO/

MOTOR DRIVE

CCD

2.94k

2k

0.1µF

C1
22µF

USB/WALL

4.5V TO 5.5V

1µF

PUSHBUTTON

MICROCONTROLLER

2

10µF

22µF

100nF100nF

15µH

10µH

VOUT3

CAP3

VFB3

IFB3

SW2

FB2

GND

CAP1

FB1

VOUT1

LT3587

EN/SS1

EN/SS3

SW3 SW1VIN

10µF

2.2µF

15µH 15µH

CCD POSITIVE

CCD NEGATIVE

8.06k

1µF

LED DRIVER
20mA, UP TO 6 LEDS

2.2µF

1M

1M

22µF

V

OUT

2.7pF

6.8pF

FLT

1µF

= IR05H40CSPTR

DESIGN IDEAS L

Linear Technology Magazine • September 2008

Figure 2. Complete power solution for portable cameras

3535

L DESIGN IDEAS

BATTERY VOLTAGE (V)

2.7

EFFICIENCY (%)

80

90

3.9

70

60

3

3.3

3.5

4.2

100

R

CLPROG

= 3.01K

R

PROG

= 1K

I

VOUT

= 0mA

5x CHARGING EFFICIENCY

1x CHARGING EFFICIENCY

10µH

VOUT3

CAP3

IFB3

LT3587

8.06k
R

IFB3

EN/SS3

SW3

VIN

1µF

V

VIN

2.5V TO 5V

V

DAC-OUT

DAC

LTC2630

PWM
FREQ

2.5V

0V

MN1
Si1304BDL

IR05H40CSPTR

Figure 3. Battery charging efficiency vs
battery voltage with no external load
(P

should be pulled-up to the same volt­age. In Figure 2 the LDO3V3 regulator
is used as the pull-up voltage for the
FAULT signal and the power supply
for the low power microcontroller
used to process pushbutton events
and sequence the power supplies.
The FAULT pin also acts as an input
and hence, must be high before any
outputs are enabled.

Compact LED Driver

The LT3587 LED driver is designed
to drive up to six LEDs with average
LED currents between 20mA and 1µA.
When the LT3587’s V
a current regulated LED driver, the
V

FB3

protection function. By connecting a
resistor between V
device limits the maximum allowable
output voltage on V
is extremely important in LED appli­cations because without it the client
device may be damaged if one of the
LEDs were to open; in such a case,
the output would continue to rise as

)

BAT/PBUS

is used as

OUT3

pin can be used as an overvoltage

and V

OUT

. This feature

OUT3

FB3

the

Figure 4. Six white LED driver with PWM and analog dimming

the current regulation loop increases
voltage in an attempt to regulate the
current.

The integrated LED driver in the
LT3587 is capable of accepting a direct
PWM dimming signal into its enable
input (EN/SS3) and/or accommodates
analog dimming via an external DAC.
See Figure 4 for a partial application
circuit showing the LED driver with
direct PWM and analog dimming.

LEDs can change color when the
current through them changes, but
PWM dimming maintains color consis­tency over the dimming range, as the
ON part of the PWM cycle is always the
same current. In PWM dimming, the
brightness of the LEDs is a function of
average current, adjusted by changing
the duty cycle of the PWM signal. In
analog dimming, the constant current
through the LEDs is adjusted, which
causes variations in color.

The LT3587 accepts PWM signals
with frequencies over 60Hz to assure
flicker-free operation. High PWM fre­quencies are achievable because of

the internal disconnect FET between
CAP3 and V

. This FET ensures

OUT3

that CAP3 maintains its steady-state
value while the PWM signal is low,
resulting in minimal startup delays.
For a 100Hz PWM dimming signal and
allowing for 10% deviation from linear­ity at the lowest duty cycle, the LT3587
allows for a dimming ratio of 30:1. If
the maximum amount of adjustment
range is desired, an external DAC,
such as the LTC2630, can be used to
feed an adjustment voltage onto the
IFB3 resistor, creating an LED current
range of 20,000:1.

Conclusion

Two highly integrated devices, the
LTC3586 and LT3587 can be combined
to create a complete USB compatible
power solution for portable cameras
and other feature-rich portable de­vices. The solution is robust, high
performance and compact, with ef­ficient battery charging, instant-on
capability and LED protection.

L

LTC3851, continued from page 17

protects against insufficient turn-on
voltage for the top MOSFET.

3.3V/15A Regulator with
DCR Sensing

Figure 2 shows a 400kHz, 3.3V output
regulator using DCR current sensing.
The DC resistance of the inductor is
used as the current sense element,
eliminating the need for a discrete
sense resistor and thus maximizing
efficiency. Figure 3 shows a plot of the

36

36

efficiency vs load for all three modes
of operation with an input voltage of
12V.

1.5V/15A Regulator
Synchronized at 350kHz

Figure 4 illustrates a 1.5V output
regulator that is synchronized to
an external clock. The loop filter
components connected to the FREQ/
PLLFLTR pin are optimized to achieve
a jitter free oscillator frequency and
reduced lock time.

Conclusion

The LTC3851 combines high perfor­mance, ease of use and a comprehensive
feature set in a 3mm × 3mm 16-pin
package. DCR current sensing and
Burst Mode® operation keep efficiency
high. With a broad 4V to 38V input
range, strong MOSFET drivers, low
minimum on-time and tracking, the
LTC3851 is ideal for automotive elec­tronics, server farms, datacom and
telecom power supply systems and
industrial equipment.

Linear Technology Magazine • September 2008

L