Rainbow Electronics MAX685 User Manual

For free samples & the latest literature: http://www.maxim-ic.com, or phone [phone].
For small orders, phone [phone] ext. 3468.

General Description

The MAX685 DC-DC converter provides low-noise dual
outputs for powering CCD imaging devices and LCDs.
This device uses a single inductor to provide indepen­dently regulated positive and negative outputs.
Integrated power switches are included in the small
16-pin QSOP package (same size as an 8-pin SO) to
save space and reduce cost.

Each output delivers up to 10mA from a +2.7V to +5.5V
input voltage range. Output voltages are set indepen­dently up to 24V and down to -9V. With a few additional
low-cost components, the output voltages can be set at
up to 45V and down to -16V. Output ripple magnitude is
30mVp-p. The MAX685 uses a fixed-frequency, pulse­width-modulated (PWM) control scheme at 220kHz or
400kHz to permit output noise filtering and to reduce
the size of external components. The frequency can
also be synchronized to an external clock signal
between 200kHz and 480kHz.

The MAX685 has a power-OK indicator output (POK)
that signals when both outputs are within regulation. A
logic-controlled shutdown completely turns off both out­puts and reduces supply current to 0.1µA. The user
can also set which output turns on first.

The preassembled MAX685 evaluation kit is available to
reduce design time.

Applications

Camcorders LCDs
Digital Cameras CCD Imaging Devices
Notebooks

Features

♦ Dual Output Using a Single Inductor
♦ Low-Noise Output, 30mVp-p Ripple
♦ Output Voltages up to 24V and down to -9V

(up to 45V and down to -16V with added
components)

♦ Internal Switches in a Small Package
♦ 220kHz/400kHz Fixed-Frequency PWM Operation
♦ Frequency Can Be Synchronized to

External Clock

♦ Power-OK Indicator
♦ Selectable Power-On Sequencing
♦ 0.1µA Logic-Controlled Shutdown

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

________________________________________________________________

Maxim Integrated Products

1

MAX685

VP
V

DD

ON

OFF

INPUT

2.7V TO 5.5V
LXN

FBP

LXP

FBN

REFGND

NEGATIVE
OUTPUT
DOWN TO -9V,
10mA

POSITIVE
OUTPUT
UP TO 24V,
10mA

SHDN

SYNC

SEQ

POK

POS

NEG

OPTIONAL

POWER-OK
INDICATOR

Typical Operating Circuit

19-1353; Rev 0; 4/98

PART

MAX685EEE -40°C to +85°C

TEMP. RANGE PIN-PACKAGE

16 QSOP

EVALUATION KIT

AVAILABLE

Pin Configuration

Ordering Information

TOP VIEW

16
15

I.C.

14

PGND

13

PGND

12

FBP
REF

11
10

FBN

9

GND

1

LXP LXN

I.C.

2

I.C. = INTERNALLY CONNECTED

VP

3
4

POK
SEQ

5

SHDN

6
7

SYNC

V

8

DD

MAX685

QSOP

MAX685

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD= VP = 5V, TA= 0°C to +85°C unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

VDD, VP to GND........................................................-0.3V to +6V

PGND to GND .......................................................-0.3V to +0.3V

V

DD

to VP...............................................................-0.3V to +0.3V

LXN, POK to GND ..................................................-0.3V to +30V

LXP to V

DD

..............................................................-15V to +0.3V

REF, SEQ,

SHDN to GND...........................-0.3V to (VDD+ 0.3V)

FBP, FBN, SYNC to GND .........................................-0.3V to +6V

Continuous Power Dissipation (T

A

= +70°C)

16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range.............................-65°C to +165°C

Lead Temperature (soldering, 10sec).............................+300°C

2.7V < VDD< 5.5V

2.7V < VDD< 5.5V

VDD= 5.5V (Note 1)

No load

VDD= VP

No load

C3 = C4 = 10µF, I

LOAD

= 5mA

I

OUT

= 0 to 10mA, C1 = 10µF

VDD= rising

VDD= 4.5V to 5.5V

V

FBP

= 1.35V, V

FBN

= -0.1V

VDD= 4.5V, V

OUT+

≥ 14.25V, V

OUT-

≤ -7.125V,

Figure 3
TA= +25°C
VDD= 4.5V

SYNC = SEQ = SHDN = GND

SYNC = V

DD

CONDITIONS

µA0.1 1Input Bias Current

V0.7 x V

DD

Logic-High Input

V0.3 x V

DD

Logic-Low Input

µA±0.01 ±0.1FBP, FBN Input Leakage Current

mV-16 10 36FBN Threshold Voltage

V1.21 1.24 1.27FBP Threshold Voltage

mV50UVLO Hysteresis

V2.35 2.5 2.65UVLO Threshold

µA0.1 10Shutdown Supply Current

V-9 -1.27Negative Output Voltage Range

VVP 24

V2.7 5.5Input Voltage Range

Positive Output Voltage Range

mVp-p30Output Voltage Ripple

%/mA0.13Load Regulation

%/V0.2Line Regulation

µA300 500Idle Quiescent Current

mA10Output Current
mA440LX Current Limit

Ω0.6 2LXP, LXN On-Resistance

mA0.8Quiescent Current

UNITSMIN TYP MAXPARAMETER

0 < I

REF

< 50µA

No load

mV-2V

REF

Load Regulation

V1.23 1.250 1.27V

REF

Output Voltage

SHUTDOWN (SHDN)

UNDERVOLTAGE LOCKOUT

REFERENCE VOLTAGE

FB INPUTS

LOGIC INPUTS (SEQ, SHDN, SYNC)

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VDD= VP = 5V, TA= 0°C to +85°C unless otherwise noted. Typical values are at TA= +25°C.)

ELECTRICAL CHARACTERISTICS

(VDD, VP = 5V, TA= -40°C to +85°C unless otherwise noted.) (Note 2)

Note 1: Negative output voltage can be larger magnitude for lower values of V

DD

. The voltage between VDDand V

OUT-

must not

exceed 14.5V.

Note 2: Specifications to -40°C are guaranteed by design, not production tested.

VDD= 5.5V (Note 1)

VDD= VP

No load

VDD= rising

VIN= 4.5V, V

OUT+

≥ 14.25V, V

OUT-

≤ -7.125V,

Figure 3
SYNC = GND

SYNC = SEQ = SHDN = GND

CONDITIONS

V-9 -1.27Negative Output Voltage Range

VVP 24

V2.7 5.5Input Voltage Range

Positive Output Voltage Range

V1.205 1.275FBP Threshold Voltage

V2.35 2.65UVLO Threshold

mA10Maximum Output Current

µA500Idle Quiescent Current

µA10Shutdown Supply Current

UNITSMIN MAXPARAMETER

CONDITIONS

SYNC = V

DD

SYNC = GND

kHz

320 400 480

Oscillator Frequency (internal)

175 220 265

kHz200 480Sync Frequency Range (external)

I

POK

= 2mA

FBN falling

FBP rising

V0.4POK Output Low Voltage

mV

UNITSMIN TYP MAX

54 79 108

PARAMETER

FBN POK Threshold

V1.090 1.122 1.150FBP POK Threshold

V

POK

= 10V µA1POK Output Off Current

2.7V < VDD≤ 5.5V

No load

V0.3 x V

DD

Logic-Low Input

mV-20 40FBN Threshold Voltage

2.7V < VDD≤ 5.5V V0.7 x V

DD

Logic-High Input

FBP rising V1.090 1.150FBP POK Threshold
FBN falling mV54 108FBN POK Threshold

SYNC INPUT

POK COMPARATORS

No load V1.225 1.275VREF Output Voltage

SHUTDOWN

UNDERVOLTAGE LOCKOUT

FB INPUTS AND REFERENCE VOLTAGE

LOGIC INPUTS (SEQ, SHDN, SYNC)

POK COMPARATORS

1.240

1.242

1.244

1.248

1.246

1.250

1.252

1.254

-40 200-20 40 60 80 100

REFERENCE VOLTAGE

vs. TEMPERATURE

MAX685-13

TEMPERATURE (°C)

REFERENCE VOLTAGE (V)

VIN = 5.0V

VIN = 3.3V

MAX685

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

4 _______________________________________________________________________________________

60

70

80

90

0 42 6 8 10

EFFICIENCY vs. LOAD CURRENT

(POSITIVE OUTPUT LOADED)

MAX685-01

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 5.0V

VIN = 3.3V

50

60

55

70

65

80

75

85

0 42 3 51 6 7 98 10

EFFICIENCY vs. LOAD CURRENT

(NEGATIVE OUTPUT LOADED)

MAX685-02

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 5.0V

VIN = 3.3V

50

65

55

60

75
70

85
80

90

0 42 6 8 10

EFFICIENCY vs. LOAD CURRENT

(BOTH OUTPUTS LOADED)

MAX685-03

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 5.0V

VIN = 3.3V

0

0.5

1.0

1.5

2.0

2.5 3.53.0 4.0 4.5 5.0 5.5

NO-LOAD CURRENT vs.

INPUT VOLTAGE

MAX685-04

INPUT VOLTAGE (V)

NO-LOAD CURRENT (mA)

150

180
170
160

190

200

210

220

230

240

250

2.7 3.73.2 4.2 4.7 5.2 5.7

LX SWITCHING FREQUENCY

vs. INPUT VOLTAGE

MAX865-07

INPUT VOLTAGE (V)

OSCILLATOR FREQUENCY (kHz)

SYNC = V

DD

1.248

1.249

1.250

0 15 205 10 25 30 35 40 45 50

REFERENCE LOAD REGULATION

MAX685-06

LOAD CURRENT (µA)

REFERENCE VOLTAGE (V)

Typical Operating Characteristics

(Circuit of Figure 3, V

OUT+

= 15V, V

OUT-

= -7.5V, TA= +25°C, unless otherwise noted.)

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

_______________________________________________________________________________________

5

11mA

5mA/div

100mV/div

1mA

I

OUT+

V

OUT+

2ms/div

V

OUT+

LOAD-TRANSIENT RESPONSE

MAX685-08

-1mA
5mA/div

100mV/div

-11mA

I

OUT-

V

OUT-

2ms/div

V

OUT-

LOAD-TRANSIENT RESPONSE

MAX685-09

100mV/div

V

OUT+

V

OUT-

VDD, VP

2V/div

100mV/div

1ms/div

INPUT 4V TO 5V, +15V AT 10mA, -7.5V AT 10mA

LINE-TRANSIENT RESPONSE

MAX685-10

5V/div

V

OUT+

V

OUT-

5V/div

2ms/div

START-UP, SEQ = LOW, V

DD

= VP = 5.0V,

+15V AT 10mA, -7.5V AT 10mA

START-UP (SEQ = LOW)

MAX685-11

5V/div

V

OUT+

V

OUT-

5V/div

2ms/div

START-UP, SEQ = HIGH, V

DD

= VP = 5.0V,

+15V AT 10mA, -7.5V AT 10mA

START-UP (SEQ = HIGH)

MAX685-12

Typical Operating Characteristics (continued)

(Circuit of Figure 3, V

OUT+

= 15V, V

OUT-

= -7.5V, TA= +25°C, unless otherwise noted.)

_______________Detailed Description

The MAX685 DC-DC converter accepts an input voltage
between +2.7V and +5.5V and generates both a positive
and negative voltage, using a single inductor (Figure 1).
It alternates between acting as a step-up converter and
as an inverting converter on a cycle-by-cycle basis. Both
output voltages are independently regulated.

Each output is separately controlled by a pulse-width­modulated (PWM) current mode regulator. This allows
the part to operate at a fixed frequency for use in noise­sensitive applications. An internal oscillator runs at
220kHz or 400kHz, or can be synchronized to an exter­nal signal. Since switching alternates between the two
regulators, each operates at half the oscillator frequency
(110kHz, 200kHz, or half the sync frequency). The oscil­lator can be synchronized to a 200kHz to 480kHz clock.

On the first cycle of operation, the part operates as a
step-up converter. LXP connects to VDD, LXN pulls to
ground, and the inductor current rises. Once the induc-

tor current rises to a level set by the positive-side error
amplifier, LXN releases and the inductor current flows
through D2 to the positive output. When the inductor
current drops to zero (which happens each cycle under
normal, discontinuous operation), LXN returns to the
input voltage.

On the second cycle, LXN is held at ground. LXP is
pulled up to the input voltage until the current reaches
the limit set by the negative error amplifier. Then LXP is
released and the inductor current flows through D1 to
the negative output. Once the inductor current reaches
zero, the voltage at LXP returns to ground. The wave­forms at LXN and LXP are shown in Figure 2 for a typi­cal pair of cycles.

The current into the LXN pin is sensed to measure the
inductor current. The MAX685 controls the inductor cur­rent to regulate both the positive and negative output
voltages.

MAX685

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

6 _______________________________________________________________________________________

Pin Description

PIN

P-Channel Switching Inductor Node. LXP turns off when the part enters shutdown.LXP1

FUNCTIONNAME

Internally Connected. Do not externally connect.I.C.2, 15

Open-Drain Power-OK Output. POK is high when both outputs are in regulation. Connect POK to VDDwith a
100kΩ pull-up resistor to VDD.

POK4

Power Input. Connect to VDD.VP3

Shutdown Input. Both outputs go to 0V in shutdown. Connect to VDDfor automatic startup.

SHDN

6

Supply Input. Bypass VDDwith a 1.0µF or greater ceramic capacitor to GND.V

DD

8

Sync Input. This pin synchronizes the oscillator to an external clock frequency between 200kHz and 480kHz.
Connect SYNC to GND (220kHz) or VDD(400kHz) for internal oscillator frequency.

SYNC7

Power-Up Sequence Select Input. Connect SEQ to GND to power the negative output voltage first. Connect
SEQ to VDDto power the positive output first.

SEQ5

GroundGND9

1.25V Reference Voltage Output. Bypass with 0.22µF to GND.REF11

Power Ground. Connect PGND to GND.PGND13, 14

Feedback for the Positive Output Voltage. Connect a resistor-divider between the positive output and GND
with the center to FBP to set the positive output voltage.

FBP12

Feedback Input for the Negative Output Voltage. Connect a resistor-divider between the negative output
and REF with the center to FBN to set the negative output voltage.

FBN10

N-Channel Switching Inductor Node. LXN pulls to GND through the internal transistor when the part is shut
down.

LXN16

SEQ and Power OK (POK)

The SEQ pin controls the power-up sequence. If SEQ is
low, the positive output is disabled until the negative
output is within 90% of its regulation point. If SEQ is
high, the negative output is disabled until the positive
output is within 90% of its regulation point. The power­OK output (POK) indicates that both output voltages

are in regulation. When both outputs are within 90% of
their regulation points, POK becomes high impedance.
Should one or both of the output voltages fall below
90% of their regulation points, POK pulls to ground.
POK can sink up to 2mA. To reduce current consump­tion, POK is high impedance while the part is in shut­down. When coming out of shutdown, POK remains
high impedance for 50ns (typ) before going low.
Connect POK to V

DD

through a 100kΩ resistor.

Synchronization/Internal

Frequency Selection

The MAX685 operates at a fixed switching frequency.
Set the operating frequency using the SYNC pin. If
SYNC is grounded, the part operates at the internally set
220kHz frequency. When SYNC is connected to VDD,
the part operates at 400kHz. The MAX685 can also be
synchronized to signals between 200kHz and 480kHz.
Note that each output switches at half the oscillator or
synchronized frequency. Since the actual switching fre­quency is one-half the applied clock signal, drive SYNC
at twice the desired switching frequency.

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

_______________________________________________________________________________________ 7

Figure 1. Functional Diagram

NEGATIVE

ERROR

AMP

TO V

OUT-

FBN

FBP

TO V

OUT+

REF

POSITIVE

ERROR

AMP

1.25V
REF

CONTROL

LOGIC

POK VP

V

DD

P

N

PGND

GND

SYNC SEQ SHDN

LXN

D2

D1

V

OUT-

V

OUT+

LXP

MAX685

Figure 2. LXN and LXP Waveforms (see also Figure 5)

LXN

LXP

+15V

0V
+5V
0V

-7.5V

MAX685

Applications Information

Figure 3 shows the standard application circuit for the
MAX685. The values shown in Table 1 will work well for
output currents up to 10mA. However, this circuit can
be optimized to a particular application by using differ­ent capacitors and a different inductor.

Higher Output Voltages

If the application requires output voltages greater than

-7.5V or +24V, use the circuit of Figure 4. This circuit
uses a charge pump to increase the output voltage
without increasing the voltage stress on the LX_ pin.
The maximum output voltages of the circuit in Figure 4
are -15V and +48V.

The voltage rating on D2, D5, and D6 must be 30V or
greater. For a larger negative output voltage without a
larger positive output (or vice versa), use one-half of
the Figure 4 circuit with one-half of the Figure 3 circuit.

Inductor Selection

A 22µH inductor is suitable for most applications.
Larger inductances will reduce inductor ripple current
and output voltage ripple, but they also typically require
larger physical size if increased resistance and losses
are not also allowed.

Small inductors are typically preferred because of
compact design and low cost. Murata LHQ and
TDK NLC types are examples of small surface-mount
inductors that work for most applications. Because these
small-size inductors use thinner wire, they exhibit higher
resistance and have greater losses than larger ones. If
the application demands higher efficiency, use larger,
lower resistance coils such as the Sumida CD43 or CD54,
Coilcraft DT1608 or DO1608, or Coiltronics UP1V series.

Filter Capacitor Selection

The output ripple voltage is a function of the peak in­ductor current, frequency, and type and value of the
output capacitors. Capacitors with low equivalent­series resistance (ESR) and large capacitance reduce
output ripple. Typically, tantalum or ceramic capacitors
are optimal. Tantalum capacitors have higher ESR and
higher capacitance than ceramic capacitors. Therefore
the ESR of tantalum capacitors determines the output
ripple, because at the frequencies used the ESR domi­nates the impedance of the capacitor. If ceramic
capacitors are used, the capacitance determines the
output ripple.

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

8 _______________________________________________________________________________________

MAX685

REF

VP V

DD

LXP

1

11

3 8

9 13, 14 16

FBN

10

POK

4

6
7

12

POK

SHDN
SYNC

C5
47pF

R1

1.0M

R5
100k

D2

NBR0520

L1

22µH

D1

NBR0520

R2

90.9k

+15V
V

OUT+

C4

2.2µF

C3

2.2µF

C2

0.22µF

C1
10µF

V

IN

R3
750k

R4
124k

-7.5V

V

OUT-

SHDN

PGND

LXNGND

SYNC

FBP

Figure 3. Standard Application Circuit

MAX685

REF

VP V

DD

LXP

C6

1µF

1

11

3 8

9 13, 14 16

C8

1µF

FBN

10

POK

4

6
7

12

POK

SHDN
SYNC

C5
47pF

R1

R5
100k

D2

D5

D6

L1

22µH

D3

D4 D1

R2

V

OUT+

C9

2.2µF

C7

2.2µF

C3

2.2µF

C2

0.22µF

C1
10µF

V

IN

R3

R4

V

OUT-

SHDN

PGND

LXNGND

SYNC

FBP

C4

2.2µF

Figure 4. Circuit for Output Voltages < -9V and > +24V

Setting the Output Voltage

The resistor-divider formed by R4 and R3 sets the neg­ative output voltage; the resistor-divider formed by R1
and R2 sets the positive output voltage. Let R4 be a
value near 100kΩ to set a resistor-divider current of
approximately 10µA. Determine the value of R3 by the
following:

Let R2 be a value near 100kΩ to set a resistor-divider
current of approximately 10µA. Determine the value of
R1 with the following formula:

R1 = R2 x (V

OUT

+ - 1.24V) / 1.24

Damping LX

LXN and LXP may ring at the conclusion of each
switching cycle when the inductor current falls to zero.
Typically the ringing waveform appears only on LX_
and has no effect on output ripple and noise. If LX_
ringing is still objectionable, it may be damped by con­necting a series RC in parallel with L1. Typically 1kΩ in
series with 100pF provides good damping with only 3%
efficiency degradation. See Figure 5.

R R

V

V

OUT

3 4

1 24

.

=

−

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

_______________________________________________________________________________________ 9

Any manufacturer

0.22µF ceramic
capacitor

C2

Any manufacturer47pF ceramic capC5

Any manufacturer

2.2µF ceramic
capacitor

C3, C4

Murata LHQ4N220J04 or
TDK NLC32522T-220K

22µH, 0.4A
inductor

L1

Motorola MBR0520LT1 (0.5A)
or Central Semiconductor
CMPSH-3

0.1A, 20V
Schottky rectifier

D1, D2

REF

Sprague 595D106X0010A2T or
AVX TAJA106K010R

10µF, 10V
tantalum cap

C1

MANUFACTURER

PART NUMBER

DESCRIPTION

Table 1. Component Values for the
Typical Operating Circuit

Figure 5. LXN and LXP Waveforms with a Series-Connected
1k

Ω

Resistor and 100pF Capacitor Connected in Parallel with

L1 to Damp Ringing

+15V

+5V
0V
+5V

0V

-7.5V

LXN

LXP

MAX685

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

10 ______________________________________________________________________________________

Package Information

QSOP.EPS

___________________Chip Information

TRANSISTOR COUNT: 902
SUBSTRATE CONNECTED TO GND

MAX685

Dual-Output (Positive and Negative),

DC-DC Converter for CCD and LCD

______________________________________________________________________________________ 11

NOTES

MAX685

Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD

NOTES

12 ______________________________________________________________________________________