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PDF MAX863 Data sheet ( Hoja de datos )
| Número de pieza | MAX863 | |
| Descripción | Dual / High-Efficiency / PFM / Step-Up DC-DC Controller | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de MAX863 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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19-1218; Rev 1; 6/97
EVALUAATVIOANILKAIBTLMEANUAL
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
_______________General Description
The MAX863 dual-output DC-DC converter contains
two independent step-up controllers in a single com-
pact package. This monolithic bi-CMOS design draws
only 85µA when both controllers are on. The input
range extends down to 1.5V, permitting use in organiz-
ers, translators, and other low-power hand-held prod-
ucts. The MAX863 provides 90% efficiency at output
loads from 20mA to over 1A. This space-saving device
is supplied in a 16-pin QSOP package that fits in the
same area as an 8-pin SOIC.
The device uses a current-limited, pulse-frequency-
modulated (PFM) control architecture that reduces start-
up surge currents and maintains low quiescent currents
for excellent low-current efficiency. Each controller
drives a low-cost, external, N-channel MOSFET switch,
whose size can be optimized for any output current or
voltage.
In larger systems, two MAX863s can be used to gener-
ate 5V, 3.3V, 12V, and 28V from just two or three bat-
tery cells. An evaluation kit (MAX863EVKIT) is available
to speed designs. For a single-output controller, refer to
the MAX608 and MAX1771 data sheets.
________________________Applications
2- and 3-Cell Portable Equipment
Organizers
Translators
Hand-Held Instruments
Palmtop Computers
Personal Digital Assistants (PDAs)
Dual Supply (Logic and LCD)
__________________Pin Configuration
TOP VIEW
SENSE1 1
VDD 2
FB1 3
BOOT 4
CS1 5
EXT1 6
GND 7
PGND 8
MAX863
QSOP
16 REF
15 SHDN2
14 LBI
13 LBO
12 FB2
11 SHDN1
10 CS2
9 EXT2
____________________________Features
o Smallest Dual Step-Up Converter: 16-Pin QSOP
o 90% Efficiency
o 1.5V Start-Up Voltage
o 85µA Max Total Quiescent Supply Current
o 1µA Shutdown Mode
o Independent Shutdown Inputs
o Drives Surface-Mount, Dual N-Channel MOSFETs
o Low-Battery Input/Output Comparator
o Step-Up/Down Configurable
______________Ordering Information
PART
MAX863C/D
MAX863EEE
TEMP. RANGE
0°C to +70°C
-40°C to +85°C
*Dice are tested at TA = +25°C.
PIN-PACKAGE
Dice*
16 QSOP
__________Typical Operating Circuit
VIN
OUT1
SENSE1 VDD BOOT
N EXT1 EXT2
CS1 CS2
OUT2
N
MAX863
LOW-BATTERY
DETECTOR OUTPUT
LBO
LBI
FB2
SHDN1
SHDN2
REF
FB1 PGND GND
ON/OFF
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
1 page  
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
______________________________________________________________Pin Description
PIN NAME
FUNCTION
1 SENSE1 Feedback Input for DC-DC Controller 1 in Fixed-Output Mode
2 VDD IC Power-Supply Input
Adjustable Feedback and Preset Output Voltage Selection Input for DC-DC Controller 1. Connect to VDD
3 FB1 for 3.3V preset output or to GND for 5V output. Connect a resistor voltage divider to adjust the output volt-
age. See the section Set the Output Voltage.
Bootstrap Low-Voltage-Oscillator Enable Input. BOOT is an active-high, logic-level input. It enables the
4
BOOT
low-voltage oscillator to allow start-up from input voltages down to 1.5V while in a bootstrapped circuit
configuration. Connect BOOT to GND when in a non-bootstrapped configuration. If BOOT is high, VDD
must be connected to OUT1.
5 CS1 Input to the Current-Sense Comparator of DC-DC Controller 1
6
EXT1
Gate-Drive Output of DC-DC Controller 1. Drives an external N-channel power MOSFET.
7
GND
Analog Ground for Internal Reference, Feedback, and Control Circuits
8 PGND High-Current Ground Return for Internal MOSFET Drivers
9
EXT2
Gate-Drive Output of DC-DC Controller 2. Drives an external N-channel power MOSFET.
10 CS2 Input to the Current-Sense Amplifier of DC-DC Controller 2
11 SHDN1 Active-Low Shutdown Input for DC-DC Controller 1. Connect to VDD for normal operation.
12
FB2
Adjustable Feedback Input for DC-DC Controller 2. Connect a resistor voltage divider to adjust the output
voltage. See the section Set the Output Voltage.
13
LBO
Low-Battery Output. An open-drain N-channel MOSFET output. Sinks current when the voltage on LBI
drops below 1.25V. If unused, connect to GND.
14
LBI
Low-Battery Comparator Input. When the voltage on LBI drops below 1.25V, LBO sinks current. If unused,
connect to GND.
15 SHDN2 Active-Low Shutdown Input for DC-DC Controller 2. Connect to VDD for normal operation.
16 REF Reference Bypass Input. Connect a 0.1µF ceramic capacitor from REF to GND.
_______________Detailed Description
The MAX863 dual, bi-CMOS, step-up, switch-mode
power-supply controller provides preset 3.3V, 5V, or
adjustable outputs. Its pulse-frequency-modulated
(PFM) control scheme combines the advantages of low
supply current at light loads and high efficiency with
heavy loads. These attributes make the MAX863 ideal
for use in portable battery-powered systems where
small size and low cost are extremely important, and
where low quiescent current and high efficiency are
needed to maximize operational battery life. Use of
external current-sense resistors and MOSFETs allows
the designer to tailor the output current and voltage
capability for a diverse range of applications.
PFM Control Scheme
Each DC-DC controller in the MAX863 uses a one-shot-
sequenced, current-limited PFM design, as shown in
Figure 1. Referring to the Typical Operating Circuit
(Figure 2) and the switching waveforms (Figures 3a–3f),
the circuit works as follows. Output voltage is sensed
by the error comparator using either an internal voltage
divider connected to SENSE1 or an external voltage
divider connected to FB1. When the output voltage
drops, the error comparator sets an internal flip-flop.
The flip-flop turns on an external MOSFET, which allows
inductor current to ramp-up, storing energy in a mag-
netic field.
_______________________________________________________________________________________ 5
5 Page 
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
minimum input voltages. Estimate the maximum input
currents for each output based on the minimum input
voltage and desired output power:
VOUT x IOUT
IIN,DC(MAX) ≅ 0.8 x VIN(MIN)
where 0.8 is chosen as a working value for the nominal
efficiency. The power source must be capable of deliv-
ering the sum of the maximum input currents of both
DC-DC converters.
Determine the Peak Switching Current
(Graphical Method)
The peak switching current set by RSENSE determines
the amount of energy transferred from the input on
each cycle. For 3.3V, 5V, 12V, and 24V output circuits,
the peak current can be selected using the output cur-
rent curves shown in Figures 5a–5d.
Determine the Peak Switching Current and
Inductance (Analytical Method)
The following boost-circuit equations are useful when
the desired output voltage differs from those listed in
Figure 5. They allow trading off peak current and induc-
tor value in consideration of component availability,
size, and cost.
Begin by calculating the minimum allowable ratio of
inductor AC ripple current to peak current, ξMIN
(Figure 6):
ξMIN =
tOFF(MIN)
tON(MAX)
x VOUT − VIN(MIN)
VIN(MIN)
where tOFF(MIN) = 2µs and tON(MAX) = 17.5µs.
Select a value for ξ greater than ξMIN. If ξMIN is less
than 1, an acceptable choice is (ξMIN + 1) / 2. If ξMIN is
greater than 1, values between ξMIN and 2 x ξMIN are
acceptable (1.5 x ξMIN, for example). Values greater
than 1 represent designs with full-load operation in dis-
continuous-conduction mode.
Now calculate the peak switching current and induc-
tance. If ξMIN ≤ ξ ≤ 1, use:
( )IPEAK
= IIN,DC MAX
x
2
2- ξ
For ξ ≥ 1%, use:
( )( )IPEAK
=
2 x IIN,DC MAX
x
VOUT + VIN x
VOUT
ξ −1
ξMIN
=
∆IL
IPEAK
∆IL
IPEAK
t
Figure 6. Ratio of Inductor AC Ripple Current to Peak Current
The suggested inductor value is:
L≅
VOUT
-
VIN(MIN)
x tOFF(MIN)
IPEAK x ξ
Round L up to the next standard inductor value.
Choose RSENSE
The peak switching current is set by RSENSE (R1 and
R2 in Figure 2):
RSENSE ≤
VCS(MIN)
IPEAK
= 85mV
IPEAK
Verify that you’ve selected the correct RSENSE by test-
ing the prototype using the minimum input voltage
while supplying the maximum output current. If the out-
put voltage droops, then decrease the value of the cur-
rent-sense resistor and adjust the other components as
necessary.
The current-sense resistor must be a small, low-induc-
tance type such as a surface-mount metal-strip resistor.
Do not use wire-wound resistors, since their high induc-
tance will corrupt the current feedback signal. In order
to allow use of standard resistor values, round RSENSE
to the next lowest value.
The current-sense resistor’s power rating should be
higher than:
( )V 2 CS MAX
RPOWER RATING = RSENSE
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet MAX863.PDF ] | |
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