General Description
The MAX1779 triple-output DC-DC converter provides
highly efficient regulated voltages required by small
active matrix, thin-film transistor (TFT) liquid-crystal displays (LCDs). One high-power DC-DC converter and
two low-power charge pumps convert the +2.7V to
+5.5V input supply voltage into three independent output voltages.
The primary high-power DC-DC converter generates a
boosted output voltage (V
MAIN
) up to 13V that is regu-
lated within ±1%. The low-power BiCMOS control circuitry and the low on-resistance (1Ω) of the integrated
power MOSFET allows efficiency up to 91%. The
250kHz current-mode pulse-width modulation (PWM)
architecture provides fast transient response and
allows the use of ultra-small inductors and ceramic
capacitors.
The dual charge pumps independently regulate one
positive output (V
POS
) and one negative output (V
NEG
).
These low-power outputs use external diode and
capacitor stages (as many stages as required) to regulate output voltages up to +40V and down to -40V. A
proprietary regulation algorithm minimizes output ripple, as well as capacitor sizes for both charge pumps.
The MAX1779 is available in the ultra-thin TSSOP package (1.1mm max height).
________________________Applications
TFT Active-Matrix LCD Displays
Passive-Matrix LCD Displays
PDAs
Digital-Still Cameras
Camcorders
Features
♦ Three Integrated DC-DC Converters
♦ 250kHz Current-Mode PWM Boost Regulator
Up to +13V Main High-Power Output
±1% Accuracy
High Efficiency (91%)
♦ Dual Charge-Pump Outputs
Up to +40V Positive Charge-Pump Output
Down to -40V Negative Charge-Pump Output
♦ Internal Supply Sequencing
♦ Internal Power MOSFETs
♦ +2.7V to +5.5V Input Supply
♦ 0.1µA Shutdown Current
♦ 0.5mA Quiescent Current
♦ Internal Soft-Start
♦ Power-Ready Output
♦ Ultra-Small External Components
♦ Thin TSSOP Package (1.1mm max)
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
________________________________________________________________ Maxim Integrated Products 1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
RDY TGND
LX
PGND
SUPP
DRVP
SUPN
DRVN
SHDN
TOP VIEW
MAX1779
TSSOP
FB
INTG
REF
IN
GND
FBP
FBN
Pin Configuration
19-1795; Rev 0; 11/00
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Ordering Information
Typical Operating Circuit appears at end of data sheet.
16 TSSOP
PIN-PACKAGETEMP. RANGE
-40°C to +85°CMAX1779EUE
PART
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= +3.0V, SHDN = IN, V
SUPP
= V
SUPN
= +10V, TGND = PGND = GND, C
REF
= 0.22µF, C
INTG
= 2200pF, TA= 0°C to +85°C,
unless otherwise noted. Typical values are at T
A
= +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.
IN, SHDN, TGND to GND .........................................-0.3V to +6V
DRVN to GND .........................................-0.3V to (V
SUPN
+ 0.3V)
DRVP to GND..........................................-0.3V to (V
SUPP
+ 0.3V)
PGND to GND.....................................................................±0.3V
RDY to GND ...........................................................-0.3V to +14V
LX, SUPP, SUPN to PGND .....................................-0.3V to +14V
INTG, REF, FB, FBN, FBP to GND...............-0.3V to (V
IN
+ 0.3V)
Continuous Power Dissipation (T
A
= +70°C)
16-Pin TSSOP (derate 9.4mW/°C above +70°C) ..........755mW
Operating Temperature Range
MAX1779EUE ..................................................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Input Supply Range V
IN
2.7 5.5 V
Input Undervoltage Threshold
VIN rising, 40mV hysteresis (typ) 2.2 2.4 2.6 V
IN Quiescent Supply Current I
IN
VFB = V
FBP
= +1.5V, V
FBN
= -0.2V 0.5 1 mA
SUPP Quiescent Current I
SUPP
V
FBP
= +1.5V
mA
SUPN Quiescent Current I
SUPN
V
FBN
= -0.1V
mA
IN Shutdown Current V
SHDN
= 0, V
IN
= +5V 0.1 10 µA
SUPP Shutdown Current V
SHDN
= 0, V
SUPP
= +13V 0.1 10 µA
SUPN Shutdown Current V
SHDN
= 0, V
SUPN
= +13V 0.1 10 µA
MAIN BOOST CONVERTER
Output Voltage Range
V
IN
13 V
FB Regulation Voltage V
FB
V
FB Input Bias Current I
FB
VFB = +1.25V, INTG = GND -50 50 nA
Operating Frequency f
OSC
Oscillator Maximum Duty Cycle
79 85 92 %
Load Regulation I
MAIN
= 0 to 50mA, V
MAIN
= +5V 0.1 %
Line Regulation 0.1
Integrator Gm
µs
LX Switch On-Resistance
)
ILX = 100mA 1.0 2.0 Ω
LX Leakage Current I
LX
VLX = +13V
20 µA
LX Current Limit I
LIM
mA
Maximum RMS LX Current
mA
FB Fault Trip Level Falling edge
1.1
V
POSITIVE CHARGE PUMP
V
SUPP
Input Supply Range
2.7 13 V
V
UVLO
0.25 0.55
0.25 0.55
V
MAIN
R
LX(ON
V
SUPP
1.235 1.248 1.261
212 250 288 kHz
320
0.01
350 450 650
250
1.07
1.14
% / V
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= +3.0V, SHDN = IN, V
SUPP
= V
SUPN
= +10V, TGND = PGND = GND, C
REF
= 0.22µF, C
INTG
= 2200pF, TA= 0°C to +85°C,
unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Operating Frequency
0.5 ×
Hz
FBP Regulation Voltage V
FBP
V
FBP Input Bias Current I
FBP
V
FBP
= +1.5V -50 50 nA
DRVP PCH On-Resistance
310Ω
V
FBP
= +1.200V 1.5 5 Ω
DRVP NCH On-Resistance
V
FBP
= +1.300V 20 kΩ
FBP Power-Ready Trip Level Rising edge
V
FBP Fault Trip Level Falling edge
V
Maximum RMS DRVP Current 0.1 A
NEGATIVE CHARGE PUMP
V
SUPN
Input Supply Range
2.7 13 V
Operating Frequency
0.5 ×
Hz
FBN Regulation Voltage V
FBN
-50 0 50
FBN Input Bias Current I
FBN
V
FBN
= -0.05V -50 50 nA
DRVN PCH On-Resistance
310Ω
V
FBN
= +0.050V 1.5 5 Ω
DRVN NCH On-Resistance
V
FBN
= -0.050V 20 kΩ
FBN Power-Ready Trip Level Falling edge 80
165
FBN Fault Trip Level Rising edge
Maximum RMS DRVN Current 0.1 A
REFERENCE
Reference Voltage V
REF
-2µA < I
REF
< 50µA
V
Reference Undervoltage
Threshold
V
REF
rising 0.9
1.2 V
LOGIC SIGNALS
SHDN Input Low Voltage 0.25V hysteresis (typ) 0.9 V
SHDN Input High Voltage 2.1 V
SHDN Input Current
1 µA
RDY Output Low Voltage I
SINK
= 2mA
0.5 V
RDY Output High Voltage V
RDY
= +13V
1 µA
V
I
SUPN
SHDN
1.20 1.25 1.30
1.09 1.13 1.16
1.231 1.25 1.269
f
OSC
1.11
f
OSC
120
140 mV
1.05
0.01
0.25
0.01
mV
mV
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
4 _______________________________________________________________________________________
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
Input Supply Range V
IN
2.7 5.5 V
Input Undervoltage Threshold
VIN rising, 40mV hysteresis (typ) 2.2 2.6 V
IN Quiescent Supply Current I
IN
VFB = V
FBP
= +1.5V, V
FBN
= -0.2V 1 mA
SUPP Quiescent Current
V
FBP
= +1.5V
mA
SUPN Quiescent Current
V
FBN
= -0.1V
mA
IN Shutdown Current V
SHDN
= 0, VIN = +5V 10 µA
SUPP Shutdown Current V
SHDN
= 0, V
SUPP
= +13V 10 µA
SUPN Shutdown Current V
SHDN
= 0, V
SUPN
= +13V 10 µA
MAIN BOOST CONVERTER
Output Voltage Range
V
IN
13 V
FB Regulation Voltage V
FB
V
FB Input Bias Current I
FB
VFB = +1.25V, INTG = GND -50 50 nA
Operating Frequency f
OSC
kHz
Oscillator Maximum Duty Cycle
79 92 %
LX Switch On-Resistance
)
I
LX
= 100mA 2.0 Ω
LX Leakage Current
I
LX
VLX = +13V 20 µA
LX Current Limit I
LIM
mA
FB Fault Trip Level Falling edge
V
POSITIVE CHARGE PUMP
SUPP Input Supply Range
2.7 13 V
FBP Regulation Voltage V
FBP
V
FBP Input Bias Current I
FBP
V
FBP
= +1.5V -50 50 nA
DRVP PCH On-Resistance 10 Ω
V
FBP
= +1.200V 5 Ω
DRVP NCH On-Resistance
V
FBP
= +1.300V 20 kΩ
FBP Power-Ready Trip Level Rising edge
V
NEGATIVE CHARGE PUMP
SUPN Input Supply Range
2.7 13 V
FBN Regulation Voltage V
FBN
-50 50 mV
FBN Input Bias Current
I
FBN
V
FBN
= -0.05V -50 50 nA
DRVN PCH On-Resistance
10 Ω
V
FBN
= +0.050V 5 Ω
DRVN NCH On-Resistance
V
FBN
= -0.050V 20 kΩ
FBN Power-Ready Trip Level Falling edge 80
mV
REFERENCE
Reference Voltage V
REF
-2µA < I
REF
< 50µA
V
Reference Undervoltage
V
REF
rising 0.9 1.2 V
ELECTRICAL CHARACTERISTICS
(VIN= +3.0V, SHDN = IN, V
SUPP
= V
SUPN
= +10V, TGND = PGND = GND, C
REF
= 0.22µF, C
INTG
= 2200pF, TA= -40°C to +85°C,
unless otherwise noted.) (Note 1)
V
UVLO
I
SUPP
I
SUPN
V
MAIN
R
LX(ON
1.225 1.271
195 305
350 700
1.07 1.14
V
SUPP
1.20 1.30
1.09 1.16
V
SUPN
1.223 1.269
0.55
0.55
165
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
_______________________________________________________________________________________ 5
4.98
4.99
5.00
5.01
5.02
0 50 100 150 200
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT
(L = 10µH, 5V OUTPUT)
MAX1779-01
I
MAIN
(mA)
V
MAIN
(V)
VIN = +3.0V
VIN = +4.2V
FIGURE 6
50
60
70
90
100
0 50 100 150 200
MAIN STEP-UP CONVERTER EFFICIENCY
vs. LOAD CURRENT
(L = 10µH, 5V OUTPUT)
MAX1779-02
I
MAIN
(mA)
EFFICIENCY (%)
VIN = +3.0V
VIN = +4.2V
80
FIGURE 6
4.98
4.99
5.00
5.01
5.02
010050 150 200 250 300
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT
(L = 33µH, 5V OUTPUT)
MAX1779-03
I
MAIN
(mA)
V
MAIN
(V)
VIN = +4.2V
VIN = +3.0V
FIGURE 5
50
60
70
80
90
100
0 10050 150 200 250 300
MAIN STEP-UP CONVERTER EFFICIENCY
vs. LOAD CURRENT
(L = 33µH, 5V OUTPUT)
MAX1779-04
I
MAIN
(mA)
EFFICIENCY (%)
VIN = +4.2V
VIN = +3.0V
FIGURE 5
9.96
9.98
10.00
10.02
10.04
0 50 100 150
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT
(L = 33µH, 10V OUTPUT)
MAX1779-05
I
MAIN
(mA)
V
MAIN
(V)
VIN = +3.3V
VIN = +5.0V
FIGURE 5
50
60
70
80
90
100
0 50 100 150
MAIN STEP-UP CONVERTER EFFICIENCY
vs. LOAD CURRENT
(L = 33µH, 10V OUTPUT)
MAX1779-06
I
MAIN
(mA)
EFFICIENCY (%)
VIN = +5.5V
VIN = +3.3V
FIGURE 5
Typical Operating Characteristics
(Circuit of Figure 5, VIN= +3.3V, TA= +25°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)
(VIN= +3.0V, SHDN = IN, V
SUPP
= V
SUPN
= +10V, TGND = PGND = GND, C
REF
= 0.22µF, C
INTG
= 2200pF, TA= -40°C to +85°C,
unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
LOGIC SIGNALS
SHDN Input Low Voltage 0.25V hysteresis (typ) 0.9 V
SHDN Input High Voltage 2.1 V
SHDN
Input Current
1 µA
RDY Output Low Voltage I
SINK
= 2mA 0.5 V
RDY Output High Leakage V
RDY
= +13V 1 µA
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
I
SHDN
4.0µs/div
RIPPLE WAVEFORMS
MAX1779-14
A. V
MAIN
= 5V, I
MAIN
= 100mA, 10mV/div
B. V
NEG
= -8V, I
NEG
= 1mA, 5mV/div
C. V
POS
= 12V, I
POS
= 1mA, 5mV/div, FIGURE 5
5V
12V
-8V
A
B
C
100µs/div
LOAD TRANSIENT
(L = 10µH, 500µs PULSE)
MAX1779-15
A. V
MAIN
= 5V, 50mV/div
B. V
MAIN
= 5mA to 50mA, 25mA/div
FIGURE 6
5.0V
0
4.9V
A
B
5.1V
50mA
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN= +3.3V, TA= +25°C, unless otherwise noted.)
11.64
11.76
12.00
11.88
12.12
12.24
0105 15202530
POSITIVE CHARGE-PUMP OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1779-10
I
POS
(mA)
V
POS
(V)
V
SUPP
= +7V
V
SUPP
= +6V
V
SUPP
= +5V
30
50
40
70
60
90
80
100
010155 202530
POSITIVE CHARGE-PUMP EFFICIENCY
vs. LOAD CURRENT
MAX1779-11
I
POS
(mA)
V
POS
(V)
V
SUPP
= +5V
V
SUPP
= +6V
V
SUPP
= +7V
V
POS
= +12V
200
220
240
260
280
300
2.5 3.53.0 4.0 4.5 5.0
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
MAX1779-12
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
1.244
1.248
1.246
1.252
1.250
1.254
1.256
02010 30 40 50
REFERENCE VOLTAGE
vs. REFERENCE LOAD CURRENT
MAX1779-13
I
REF
(µA)
V
REF
(V)
50
60
70
80
90
100
0 10050 150 200 250
EFFICIENCY vs. LOAD CURRENT
(BOOST CONVERTER AND CHARGE PUMPS)
MAX1779-07
I
MAIN
(mA)
EFFICIENCY (%)
V
MAIN
= +5V
TWO-STAGE
CHARGE PUMPS
V
MAIN
= +10V
SINGLE-STAGE
CHARGE PUMPS
V
NEG
= -8V, I
NEG
= 1mA
V
POS
= +12V, I
POS
= 1mA
-8.08
-8.04
-8.00
-7.96
-7.92
-7.88
-7.84
-7.80
-7.76
0 5 10 15 20
NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1779-08
I
NEG
(mA)
V
NEG
(V)
V
SUPN
= +5V
V
SUPN
= +6V
V
SUPN
= +7V
30
50
40
70
60
90
80
100
NEGATIVE CHARGE-PUMP EFFICIENCY
vs. LOAD CURRENT
MAX1779-09
I
NEG
(mA)
EFFICIENCY (%)
0 5 10 15 20
V
SUPN
= +5V
V
NEG
= -8V
V
SUPN
= +6V
V
SUPN
= +7V
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
_______________________________________________________________________________________ 7
100µs/div
LOAD TRANSIENT WITHOUT INTEGRATOR
(L = 10µH, 500µs PULSE)
MAX1779-16
A. V
MAIN
= 5V, 50mV/div
B. V
MAIN
= 5mA to 50mA, 25mA/div
INTG = REF, FIGURE 6
5.0V
0
4.9V
A
B
50mA
10µs/div
LOAD TRANSIENT WITHOUT INTEGRATOR
(L = 10µH, 5µs PULSE)
MAX1779-17
A. V
MAIN
= 5V, 100mV/div
B. I
L
, 200mA/div
C. I
MAIN
= 10mA to 100mA, 100mA/div
INTG = REF, FIGURE 6
5.0V
0
400mA
A
B
100mA
200mA
0
C
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN= +3.3V, TA= +25°C, unless otherwise noted.)
100µs/div
LOAD TRANSIENT
(L = 33µH, 500µs PULSE)
MAX1779-18
A. V
MAIN
= 5V, 50mV/div
B. I
MAIN
= 10mA to 100mA, 50mA/div
FIGURE 5
5.1V
0
A
B
4.9V
100mA
5V
100µs/div
LOAD TRANSIENT WITHOUT INTEGRATOR
(L = 33µH, 500µs PULSE)
MAX1779-19
A. V
MAIN
= 5V, 50mV/div
B. I
MAIN
= 10mA to 100mA, 50mA/div
INTG = REF, FIGURE 5
5.1V
0
A
B
4.9V
100mA
5.0V
10µs/div
LOAD TRANSIENT
(L = 33µH, 5µs PULSE)
MAX1779-20
A. V
MAIN
= 5V, 50mV/div
B. I
MAIN
= 20mA to 200mA, 100mA/div
FIGURE 5
5.1V
0
A
B
4.9V
200mA
5.0V
200µs/div
STARTUP WAVEFORM
(L = 10µH)
MAX1779-21
A. V
SHDN
= 0 to 2V, 2V/div
B. V
MAIN
= 5V, 1V/div
C. I
L
, 500 mA/div
FIGURE 6, R
MAIN
= 100Ω
2V
0
A
B
3V
500mA
0
5V
C
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 RDY Active-Low Open-Drain Output. Indicates all outputs are ready. The on-resistance is 125Ω (typ).
2FB
Main Boost Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback resistive
divider to analog ground (GND).
3 INTG
Main Boost Integrator Output. If used, connect 2200pF to analog ground (GND). To disable
integrator, connect to REF.
4IN
Supply Input. +2.7V to +5.5V input range. Bypass with a 0.1µF capacitor between IN and GND, as
close to the pins as possible.
5 GND Analog Ground. Connect to power ground (PGND) underneath the IC.
6 REF
Internal Reference Bypass Terminal. Connect a 0.22µF capacitor from this terminal to analog ground
(GND). External load capability to 50µA.
7 FBP
Positive Charge-Pump Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback
resistive divider to analog ground (GND).
8 FBN Negative Charge-Pump Regulator Feedback Input. Regulates to 0V nominal.
9
SHDN
Active-Low Logic-Level Shutdown Input. Connect SHDN to IN for normal operation.
Typical Operating Characteristics (continued)
(Circuit of Figure 5, VIN= +3.3V, TA= +25°C, unless otherwise noted.)
4ms/div
POWER-UP SEQUENCING
MAX1779-23
A. V
SHDN
= 0 to 2V, 2V/div
B. V
MAIN
= 5V, R
MAIN
= 50Ω, 2.5V/div
C. V
NEG
= -8V, R
NEG
= 8kΩ, 10V/div
D. V
POS
= +12V, R
POS
= 12kΩ, 10V/div
2V
10V
A
B
0
-10V
0
5V
C
0
D
200µs/div
STARTUP WAVEFORM
(L = 33µH)
MAX1779-22
A. V
SHDN
= 0 to 2V, 2V/div
B. V
MAIN
= 5V, 1V/div
C. I
L
, 500mA/div
R
MAIN
= 50Ω
2V
0
A
B
3V
500mA
0
5V
C
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
_______________________________________________________________________________________ 9
Detailed Description
The MAX1779 is a highly efficient triple-output power
supply for TFT LCD applications. The device contains
one high-power step-up converter and two low-power
charge pumps. The primary boost converter uses an
internal N-channel MOSFET to provide maximum efficiency and to minimize the number of external components. The output voltage of the main boost converter
(V
MAIN
) can be set from VINto 13V with external resistors.
The dual charge pumps independently regulate a positive output (V
POS
) and a negative output (V
NEG
). These
low-power outputs use external diode and capacitor
stages (as many stages as required) to regulate output
voltages up to +40V and down to -40V. A proprietary
regulation algorithm minimizes output ripple as well as
capacitor sizes for both charge pumps.
Also included in the MAX1779 are a precision 1.25V
reference that sources up to 50µA, logic shutdown,
soft-start, power-up sequencing, fault detection, and an
active-low open-drain ready output.
Main Boost Converter
The MAX1779 main step-up converter switches at a
constant 250kHz internal oscillator frequency to allow
the use of small inductors and output capacitors. The
MOSFET switch pulse width is modulated to control the
power transferred on each switching cycle and to regulate the output voltage.
During PWM operation, the internal clock’s rising edge
sets a flip-flop, which turns on the N-channel MOSFET
(Figure 1). The switch turns off when the voltage-error,
slope-compensation, and current-feedback signals trip
the comparators and reset the flip-flop. The switch
remains off for the rest of the clock cycle. Changes in
the output voltage error signal shift the switch current
trip level, consequently modulating the MOSFET duty
cycle.
Dual Charge-Pump Regulator
The MAX1779 contains two individual low-power charge
pumps. One charge pump inverts the supply voltage
(SUPN) and provides a regulated negative output voltage.
The second charge pump doubles the supply voltage
(SUPP) and provides a regulated positive output voltage.
The MAX1779 contains internal P-channel and N-channel
MOSFETs to control the power transfer. The internal
MOSFETs switch at a constant 125kHz (0.5 ✕f
OSC
).
Negative Charge Pump
During the first half-cycle, the P-channel MOSFET turns
on and the flying capacitor C5 charges to V
SUPN
minus
a diode drop (Figure 2). During the second half-cycle,
the P-channel MOSFET turns off, and the N-channel
MOSFET turns on, level shifting C5. This connects C5 in
parallel with the reservoir capacitor C6. If the voltage
across C6 minus a diode drop is lower than the voltage
across C5, charge flows from C5 to C6 until the diode
(D5) turns off. The amount of charge transferred to the
output is controlled by the variable N-channel on-resistance.
Positive Charge Pump
During the first half-cycle, the N-channel MOSFET turns
on and charges the flying capacitor C3 (Figure 3). This
initial charge is controlled by the variable N-channel
on-resistance. During the second half-cycle, the Nchannel MOSFET turns off and the P-channel MOSFET
turns on, level shifting C3 by V
SUPP
volts. This connects
C3 in parallel with the reservoir capacitor C4. If the voltage across C4 plus a diode drop (V
POS
+ V
DIODE
) is
smaller than the level-shifted flying capacitor voltage
Pin Description (continued)
PIN NAME FUNCTION
10 DRVN Negative Charge-Pump Driver Output. Output high level is V
SUPN
, and low level is PGND.
11 SUPN Negative Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor.
12 DRVP Positive Charge-Pump Driver Output. Output high level is V
SUPP
, and low level is PGND.
13 SUPP Positive Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor.
14 PGND Power Ground. Connect to GND underneath the IC.
15 LX
Main Boost Regulator Power MOSFET N-Channel Drain. Connect output diode and output capacitor
as close to PGND as possible.
16 TGND Must be connected to ground.
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
10 ______________________________________________________________________________________
(VC3+ V
SUPP
), charge flows from C3 to C4 until the
diode (D3) turns off.
Soft-Start
The main boost regulator does not have soft-start.
For the charge pumps, soft-start is achieved by controlling the rise rate of the output voltage. The output voltage regulates within 16ms, regardless of output
capacitance and load, limited only by the regulator’s
output impedance (see the Startup Waveforms in the
Typical Operating Characteristics).
Shutdown
A logic-low level on SHDN disables all three MAX1779
converters and the reference. When shut down, the
supply current drops to 0.1µA to maximize battery life
and the reference is pulled to ground. The output
capacitance and load current determine the rate at
which each output voltage will decay. A logic-level high
on SHDN activates the MAX1779 (see Power-Up
Sequencing). Do not leave SHDN floating. If unused,
connect SHDN to IN.
Power-Up Sequencing
Upon power-up or exiting shutdown, the MAX1779
starts a power-up sequence. First, the reference powers up. Then the main DC-DC step-up converter powers up. Once the main boost converter reaches
regulation, the negative charge pump turns on. When
the negative output voltage reaches approximately 90%
of its nominal value (V
FBN
< 120mV), the positive
charge pump starts up. Finally, when the positive output voltage reaches 90% of its nominal value (V
FBP
>
LX
GND
PGND
1.25V
R2
V
IN
= 2.7V TO 5.5V
L1
IN
R1
Q
R
S
OSC
REF
FB
V
MAIN
(UP TO 13V)
V
OUT =
[1 + (R1 / R2)] x V
REF
V
REF
= 1.25V
I
LIM
C1
C2
C
COMP
R
COMP
D1
INTG
+
+
-
+
-
-
MAX1779
Gm
-
+
-
C
INTG
+
-
SLOPE
COMP
+
+
Σ
Figure 1. PWM Boost Converter Block Diagram
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
______________________________________________________________________________________ 11
GND
PGND
R6
C
REF
0.22µF
V
SUPN
= 2.7V TO 13V
SUPN
OSC
R5
V
NEG
C5
C6
D5
D4
+
-
DRVN
FBN
REF
MAX1779
+
-
V
REF
1.25V
V
NEG
= - V
REF
V
REF
= 1.25V
R5
R6
( )
Figure 2. Negative Charge-Pump Block Diagram
GND
PGND
V
REF
1.25V
R4
V
SUPP
= 2.7V TO 13V
SUPP
OSC
R3
V
POS
C3
C4
D3
D2
+
-
DRVP
FBP
MAX1779
+
-
V
POS
= 1 + V
REF
V
REF
= 1.25V
R3
R4
( )
Figure 3. Positive Charge-Pump Block Diagram
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
12 ______________________________________________________________________________________
1.125V), the active-low ready signal (RDY) is pulled low
(see Power Ready section).
Power Ready
Power ready is an open-drain output. When the powerup sequence is properly completed, the MOSFET turns
on and pulls RDY low with a typical 125Ω on-resis-
tance. If a fault is detected, the internal open-drain
MOSFET appears as a high impedance. Connect a
100kΩ pullup resistor between RDY and IN for a logiclevel output.
Fault Detection
Once RDY is low, if any output falls below its faultdetection threshold, then RDY becomes high impedance.
For the reference, the fault threshold is 1.05V. For the
main boost converter, the fault threshold is 88% of its
nominal value (VFB< 1.1V). For the negative charge
pump, the fault threshold is approximately 88% of its
nominal value (V
FBN
< 140mV). For the positive charge
pump, the fault threshold is 88% of its nominal value
(V
FBP
< 1.11V).
Once an output faults, all outputs later in the power
sequence shut down until the faulted output rises
above its power-up threshold. For example, if the negative charge-pump output voltage falls below the fault
detection threshold, the main boost converter remains
active while the positive charge pump stops switching
and its output voltage decays, depending on output
capacitance and load. The positive charge-pump output will not power up until the negative charge-pump
output voltage rises above its power-up threshold (see
the Power-Up Sequencing section).
Voltage Reference
The voltage at REF is nominally 1.25V. The reference
can source up to 50µA with good load regulation (see
Typical Operating Characteristics). Connect a 0.22µF
bypass capacitor between REF and GND.
Design Procedure
Main Boost Converter
Inductor Selection
Inductor selection depends upon the minimum required
inductance value, saturation rating, series resistance,
and size. These factors influence the converter’s efficiency, maximum output load capability, transient
response time, and output voltage ripple. For most
applications, values between 10µH and 33µH work
best with the controller’s switching frequency.
The inductor value depends on the maximum output
load the application must support, input voltage, and
output voltage. With high inductor values, the MAX1779
sources higher output currents, has less output ripple,
and enters continuous-conduction operation with lighter
loads; however, the circuit’s transient response time is
slower. On the other hand, low-value inductors respond
faster to transients, remain in discontinuous-conduction
operation, and typically offer smaller physical size. The
maximum output current an inductor value will support
may be calculated by the following equations:
A. Continuous-conduction: if
then
B. Discontinuous-conduction: if
then
where I
LIM(MIN)
= 350mA and ƒ = 250kHz (see the
Electrical Characteristics).
The inductor’s saturation current rating should exceed
peak inductor current throughout the normal operating
range. Under fault conditions, the inductor current may
reach up to 600mA (I
LIM(MAX)
, see the Electrical
Characteristics). However, the MAX1779’s fast currentlimit circuitry allows the use of soft-saturation inductors
while still protecting the IC.
The inductor’s DC resistance significantly affects efficiency due to the power loss in the inductor. The power
loss due to the inductor’s series resistance (PLR) may
be approximated by the following equation:
P
IV
V
R
LR
MAIN MAIN
IN
L
≅
×
×
2
L
IVV
I
MAIN MAX MAIN IN MIN
LIM MIN
≥
ƒ
()
2
1
2
() ()
()
-
I
V
V
I
MAIN MAX
IN MIN
MAIN
LIM MIN()
()
()
<
1
2
L
V
V
IN MIN
MAIN
V
V
V
II
MAIN
V
IN MIN
IN MIN
MAIN
LIM MIN MAIN MAX
≥
ƒ
121
2
()
()
()
() ( )
-
-
I
V
V
I
MAIN MAX
IN MIN
MAIN
LIM MIN()
()
()
≥
1
2
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
______________________________________________________________________________________ 13
where RL is the inductor’s series resistance. For best
performance, select inductors with resistance less than
the internal N-channel MOSFET on-resistance (1Ω typ).
Output Capacitor
The output capacitor selection depends on circuit stability and output voltage ripple. In order to deliver the
maximum output current capability of the MAX1779, the
inductor must run in continuous-conduction mode (see
Inductor Selection). The minimum recommended output
capacitance is:
For configurations that need less output current, the
MAX1779 allows lower output capacitance when operating in discontinuous-conduction mode throughout the
load range. Under these conditions, at least 10µF is
recommended, as shown in Figure 6. In both discontinuous and continuous operation, additional feedback
compensation is required (see the Feedback
Compensation section) to increase the margin for stability by reducing the bandwidth further. In cases where
the output capacitance is sufficiently large, additional
feedback compensation will not be necessary.
However, in certain applications that require benign
load transients and constantly operate in discontinuous-conduction mode, output capacitance less than
10µF may be used.
Output voltage ripple has two components: variations in
the charge stored in the output capacitor with each LX
pulse, and the voltage drop across the capacitor’s
equivalent series resistance (ESR) caused by the current into and out of the capacitor:
V
RIPPLE
= V
RIPPLE(C)
+ V
RIPPLE(ESR)
For low-value ceramic capacitors, the output voltage
ripple is dominated by V
RIPPLE(C)
.
Integrator Capacitor
The MAX1779 contains an internal current integrator
that improves the DC load regulation but increases the
peak-to-peak transient voltage (see the Load Transient
Waveforms in the Typical Operating Characteristics).
For highly accurate DC load regulation, enable the integrator by connecting a capacitor to INTG. The minimum
capacitor value should be C
OUT
/10k or 1nF, whichever
is greater. Alternatively, to minimize the peak-to-peak
transient voltage at the expense of DC load regulation,
disable the integrator by connecting INTG to REF and
adding a 100kΩ resistor to GND.
Feedback Compensation
Compensation on the feedback node is required to
have enough margin for stability. Add a pole-zero pair
from FB to GND in the form of a compensation resistor
(R
COMP
in Figures 5 and 6) in series with a compensa-
tion capacitor (C
COMP
in Figures 5 and 6). For continu-
ous conduction operation, select R
COMP
to be 1/2 the
value of R2, the low-side feedback resistor. For discontinuous-conduction operation, select R
COMP
to be 1/5th
the value of R2.
Start with a compensation capacitor value of (220pF
✕
R
COMP
)/10kΩ. Increase this value to improve the DC
stability as necessary. Larger compensation values
slow down the converter’s response time. Check the
startup waveform for excessive overshoot each time the
compensation capacitor value is increased.
Charge Pump
Efficiency Considerations
The efficiency characteristics of the MAX1779 regulated
charge pumps are similar to a linear regulator. They are
dominated by quiescent current at low output currents
and by the input voltage at higher output currents (see
Typical Operating Characteristics). So the maximum
efficiency may be approximated by:
Efficiency ≅IV
NEG
I
/ [V
IN
✕
N];
for the negative charge pump
Efficiency ≅ V
POS
/ [V
IN
✕
(N + 1)];
for the positive charge pump
where N is the number of charge-pump stages.
Output Voltage Selection
Adjust the positive output voltage by connecting a voltage-divider from the output (V
POS
) to FBP to GND (see
Typical Operating Circuit). Adjust the negative output
voltage by connecting a voltage-divider from the output
(V
NEG
) to FBN to REF. Select R4 and R6 in the 50kΩ to
100kΩ range. Higher resistor values improve efficiency
at low output current but increase output voltage error
due to the feedback input bias current. Calculate the
remaining resistors with the following equations:
R3 = R4 [(V
POS
/ V
REF
) - 1]
R5 = R6 (IV
NEG
/ V
REF
I
)
where V
REF
= 1.25V. V
POS
may range from V
SUPP
to
+40V, and V
NEG
may range from 0 to -40V.
Flying Capacitor
Increasing the flying capacitor’s value increases the
output current capability. Above a certain point,
increasing the capacitance has a negligible effect
because the output current capability becomes domi-
C
LI
VV
OUT
MAIN MAX
MAIN IN MIN
>
××
×
60
()
()
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
14 ______________________________________________________________________________________
nated by the internal switch resistance and the diode
impedance. Start with 0.1µF ceramic capacitors.
Smaller values may be used for low-current applications.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-topeak transient voltage. Use the following equation to
approximate the required capacitor value:
C
PUMP
≥ [I
PUMP
/ (125kHz ✕V
RIPPLE
)]
Charge-Pump Input Capacitor
Use a bypass capacitor with a value equal to or greater
than the flying capacitor. Place the capacitor as close
to the IC as possible. Connect directly to PGND.
Rectifier Diode
Use Schottky diodes with a current rating equal to or
greater than 4 times the average output current, and a
voltage rating at least 1.5 times V
SUPP
for the positive
charge pump and V
SUPN
for the negative charge pump.
PC Board Layout and Grounding
Carefully printed circuit layout is extremely important to
minimize ground bounce and noise. First, place the
main boost converter output diode and output capacitor
less than 0.2in (5mm) from the LX and PGND pins with
wide traces and no vias. Then place 0.1µF ceramic
bypass capacitors near the charge-pump input pins
(SUPP and SUPN) to the PGND pin. Keep the chargepump circuitry as close to the IC as possible, using
wide traces and avoiding vias when possible. Locate
all feedback resistive dividers as close to their respective feedback pins as possible. The PC board should
feature separate GND and PGND areas connected at
only one point under the IC. To maximize output power
and efficiency and to minimize output power ripple voltage, use extra wide power ground traces and solder
the IC’s power ground pin directly to it. Avoid having
sensitive traces near the switching nodes and high-current lines.
Refer to the MAX1779 evaluation kit for an example of
proper board layout.
Applications Information
LX Charge Pump
Some applications require multiple charge-pump
stages due to low supply voltages. In order to reduce
the circuit’s size and component count, an unregulated
charge pump may be added onto the LX switching
node. The configuration shown in Figure 4 works well
for most applications. The maximum output current of
the low-power charge pumps depends on the maxi-
mum load current that the LX charge pump can provide
and is limited by the following formula:
I
LXPUMP
= ((N + 1) ✕I
POS
) + (M + I
NEG
) ≤ 5mA
where N is the number of stages in the positive lowpower charge pump, and M is the number of stages in
the negative charge pump. Applications requiring more
output current should not use the LX charge pump, so
they will require extra stages on both low-power charge
pumps. The output capacitor of this unregulated
charge pump needs to be stacked on top of the main
output in order to keep the main regulator stable.
Increasing the integrator capacitor may also be
required to compensate for the additional charge-pump
capacitance on the main regulator loop.
The output capacitor of this unregulated charge pump
needs to be stacked on top of the main output in order
to keep the main regulator stable. Increasing the integrator capacitor may also be required to compensate
for the additional charge-pump capacitance on the
main regulator loop.
Chip Information
TRANSISTOR COUNT: 2846
SUPPLIER PHONE FAX
INDUCTORS
Coilcraft 847-639-6400 847-639-1469
Coiltronics 561-241-7876 561-241-9339
Sumida USA 847-956-0666 847-956-0702
Toko 847-297-0070 847-699-1194
CAPACITORS
AVX 803-946-0690 803-626-3123
Kemet 408-986-0424 408-986-1442
Sanyo 619-661-6835 619-661-1055
Taiyo Yuden 408-573-4150 408-573-4159
DIODES
Central
Semiconductor
516-435-1110 516-435-1824
International
Rectifier
310-322-3331 310-322-3332
Motorola 602-303-5454 602-994-6430
Nihon 847-843-7500 847-843-2798
Zetex 516-543-7100 516-864-7630
Table 1. Component Suppliers
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
______________________________________________________________________________________ 15
C
REF
0.22µF
C
INTG
3300pF
R4
49.9k
R3
549k
0.1µF
10µH
V
IN
= +3.0V
SHDN
RDY
V
POS
=
+15V, 1mA
R2
50k
R1
150k
R
COMP
10k
C
COMP
220pF
V
MAIN
= +5V
C
OUT
(2) 4.7µF
IN LX
FB
SUPN
SUPP
DRVP
FBP
PGND
DRVN
INTG
TGND
GND
MAX1779
0.1µF
1.0µF
R5
320k R6
49.9k
V
NEG
= -8V, 1mA
FBN
REF
0.1µF
1.0µF
(2) 4.7µF
100k
1.0µF
0.47µF
Figure 4. Minimizing the Number of Charge-Pump Stages
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
16 ______________________________________________________________________________________
V
IN
= +3.3V
C
IN
10µF
R
RDY
100k
C11
0.1µF
IN
SHDN
RDY
C
INTG
2200pF
C5
0.1µF
DRVN
C9
0.22µF
C6
0.47µF
R5
320k
V
NEG
-8V, 5mA
C10
2.2µF
R6
49.9k
C
REF
0.22µF
PGND
REF
FBN
GND
TGND
FBP
DRVP
SUPP
SUPN
FB
LX
R1
150k
R
COMP
24k
C
COMP
470pF
R2
50k
V
MAIN
= +5.0V
C
OUT
22µF
C3
0.1µF
C3
0.47µF
VPOS
+12V, 5mA
C8
2.2µF
C8
430k
R4
49.9k
C7
0.22µF
MAX1779
INTG
33µH
Figure 5. Typical Operating Circuit (L = 33µH)
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
______________________________________________________________________________________ 17
IN
SHDN
RDY
INTG
LX
FB
SUPN
SUPP
DRVN
DRVP
GND
TGND
FBP
PGND
REF
FBN
R1
150k
R
COMP
10k
C
COMP
220pF
R2
50k
C
OUT
(2) 4.7µF
V
MAIN
= +5.0V
C3
0.1µF
C4
0.47µF
V
POS
+12V, 5mA
C8
2.2µF
R3
430k
R4
49.9k
C7
0.22µF
10µH
C11
0.1µF
R
RDY
100k
C
IN
(2) 4.7µF
V
IN
= +3.3V
C
INTG
2200pF
C5
0.1µF
C9
0.22µF
C6
0.47µF
VNEG
-8V, 5mA
C10
2.2µF
R5
320k
R6
49.9k
C
REF
0.22µF
MAX1779
Figure 6. Typical Operating Circuit (L = 10µH)
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
TSSOP.EPS
Note: The MAX1779 16-pin TSSOP package does not have an exposed pad.
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