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MAX4193ESA+

P1-P3P4-P6P7-P9P10-P12P13-P14
Potted Toroidal Inductors
A typical 1mH, 0.82Ω potted toroidal inductor (Dale TE-
3Q4TA) is 0.685in in diameter by 0.385in high and
mounts directly onto a PC board by its leads. Such
devices offer high efficiency and mounting ease, but at
a somewhat higher cost than molded inductors.
Ferrite Cores (Pot Cores)
Pot cores are very popular as switch-mode inductors
since they offer high performance and ease of design.
The coils are generally wound on a plastic bobbin,
which is then placed between two pot core sections. A
simple clip to hold the core sections together com-
pletes the inductor. Smaller pot cores mount directly
onto PC boards through the bobbin terminals. Cores
come in a wide variety of sizes, often with the center
posts ground down to provide an air gap. The gap pre-
vents saturation while accurately defining the induc-
tance per turn squared.
Pot cores are suitable for all DC-DC converters, but are
usually used in the higher power applications. They are
also useful for experimentation since it is easy to wind
coils onto the plastic bobbins.
Toroidal Cores
In volume production, the toroidal core offers high per-
formance, low size and weight, and low cost. They are,
however, slightly more difficult for prototyping, in that
manually winding turns onto a toroid is more tedious
than on the plastic bobbins used with pot cores.
Toroids are more efficient for a given size since the flux
is more evenly distributed than in a pot core, where the
effective core area differs between the post, side, top,
and bottom.
Since it is difficult to gap a toroid, manufacturers produce
toroids using a mixture of ferromagnetic powder (typically
iron or Mo-Permalloy powder) and a binder. The perme-
ability is controlled by varying the amount of binder,
which changes the effective gap between the ferromag-
netic particles. Mo-Permalloy powder (MPP) cores have
lower losses and are recommended for the highest effi-
ciency, while iron powder cores are lower cost.
Diodes
In most MAX630 circuits, the inductor current returns to
zero before L
X
turns on for the next output pulse. This
allows the use of slow turn-off diodes. On the other
hand, the diode current abruptly goes from zero to full
peak current each time L
X
switches off (Figure 1, D1).
To avoid excessive losses, the diode must therefore
have a fast turn-on time.
For low-power circuits with peak currents less than
100mA, signal diodes such as 1N4148s perform well.
For higher-current circuits, or for maximum efficiency at
low power, the 1N5817 series of Schottky diodes are
recommended. Although 1N4001s and other general-
purpose rectifiers are rated for high currents, they are
unacceptable because their slow turn-on time results in
excessive losses.
MAX630/MAX4193
CMOS Micropower Step-Up
Switching Regulator
_______________________________________________________________________________________ 7
MANUFACTURER TYPICAL PART NUMBER DESCRIPTION
MOLDED INDUCTORS
Dale IHA-104 500µH, 0.5Ω
Nytronics WEE-470 470µH, 10Ω
TRW LL-500 500µH, 0.75Ω
POTTED TOROIDAL INDUCTORS
Dale TE-3Q4TA 1mH, 0.82Ω
TRW MH-1 600µH, 1.9Ω
Torotel Prod. PT 53-18 500µH, 5Ω
FERRITE CORES AND TOROIDS
Allen Bradley T0451S100A Tor. core, 500nH/T
2
Siemens B64290-K38-X38 Tor. core, 4µH/T
2
Magnetics 555130 Tor. core, 53nH/T
2
Stackpole 57-3215 Pot core, 14mm x 18mm
Magnetics G-41408-25 Pot core, 14 x 8, 250nH/T
2
Table 1. Coil and Core Manufacturers
Note: This list does not constitute an endorsement by Maxim Integrated Products and is not intended to be a comprehensive list of
all manufacturers of these components.
MAX630/MAX4193
Filter Capacitor
The output-voltage ripple has two components, with
approximately 90 degrees phase difference between
them. One component is created by the change in the
capacitor’s stored charge with each output pulse. The
other ripple component is the product of the capacitor’s
charge/discharge current and its effective series resis-
tance (ESR). With low-cost aluminum electrolytic
capacitors, the ESR-produced ripple is generally larger
than that caused by the change in charge.
where V
IN
is the coil input voltage, L is its inductance, f
is the oscillator frequency, and ESR is the equivalent
series resistance of the filter capacitor.
The output ripple resulting from the change in charge
on the filter capacitor is:
where t
CHG
and t
DIS
are the charge and discharge
times for the inductor (1/2f can be used for nominal cal-
culations).
Oscillator Capacitor, C
X
The oscillator capacitor, C
X
, is a noncritical ceramic or
silver mica capacitor. C
X
can also be calculated by:
where f is the desired operating frequency in Hertz, and
C
INT
is the sum of the stray capacitance on the C
X
pin
and the internal capacitance of the package. The internal
capacitance is typically 1pF for the plastic package and
3pF for the CERDIP package. Typical stray capacitances
are about 3pF for normal PC board layouts, but will be
significantly higher if a socket is used.
Bypassing and Compensation
Since the inductor-charging current can be relatively
large, high currents can flow through the ground con-
nection of the MAX630/MAX4193. To prevent unwanted
feedback, the impedance of the ground path must be
as low as possible, and supply bypassing should be
used for the device.
When large values (>50kΩ) are used for the voltage-
setting resistors, R1 and R2 of Figure 1, stray capaci-
tance at the V
FB
input can add a lag to the feedback
response, destabilizing the regulator, increasing low-
frequency ripple, and lowering efficiency. This can
often be avoided by minimizing the stray capacitance
at the V
FB
node. It can also be remedied by adding a
lead compensation capacitor of 100pF to 10nF in paral-
lel with R1 in Figure 1.
DC-DC Converter Configurations
DC-DC converters come in three basic topologies:
buck, boost, and buck-boost (Figure 2). The MAX630 is
usually operated in the positive-voltage boost circuit,
where the output voltage is greater than the input.
The boost circuit is used where the input voltage is
always less than the desired output and the buck circuit
is used where the input is greater than the output. The
buck-boost circuit inverts, and can be used with, input
C
X
f
C C pF seetext
X INT INT
=−
−
≅
214 10
5
6
.
(,)
V
Q
C
where Q t x
I
and I t x
V
L
V
Vt t
LC
dQ DIS
PEAK
PEAK CHG
IN
dQ
IN CHG DIS
==
=
=
,
,
()()
2
2
V I x ESR
V
Lf
xESR Voltsp p
ESR PK
IN
==
⎛
⎝
⎜
⎞
⎠
⎟
−
2
()
CMOS Micropower Step-Up
Switching Regulator
8 _______________________________________________________________________________________
CONTROL
SECTION
V
BATT
S
1
V
OUT
> V
BATT
+
-
BOOST CONVERTER
CONTROL
SECTION
V
BATT
S
1
V
OUT
< V
BATT
+
-
BUCK CONVERTER
CONTROL
SECTION
V
BATT
S
1
|V
OUT
| < OR > V
BATT
+
-
BUCK-BOOST CONVERTER
Figure 2. DC-DC Converter Configurations
voltages that are either greater or less than the output.
DC-DC converters can also be classified by the control
method. The two most common are pulse-width modu-
lation (PWM) and pulse-frequency modulation (PFM).
PWM switch-mode power-supply ICs (of which current-
mode control is one variant) are well-established in
high-power off-line switchers. Both PWM and PFM cir-
cuits control the output voltage by varying duty cycle.
In the PWM circuit, the frequency is held constant and
the width of each pulse is varied. In the PFM circuit, the
pulse width is held constant and duty cycle is con-
trolled by changing the pulse repetition rate.
The MAX630 refines the basic PFM by employing a con-
stant-frequency oscillator. Its output MOSFET is switched
on when the oscillator is high and the output voltages is
lower than desired. If the output voltage is higher than
desired, the MOSFET output is disabled for that oscillator
cycle. This pulse skipping varies the average duty cycle,
and thereby controls the output voltage.
Note that, unlike the PWM ICs, which use an op amp as
the control element, the MAX630 uses a comparator to
compare the output voltage to an onboard reference.
This reduces the number of external components and
operating current.
Typical Applications
+5V to +15V DC-DC Converter
Figure 1 shows a simple circuit that generates +15V at
approximately 20mA from a +5V input. The MAX630
has a ±1.5% reference accuracy, so the output voltage
has an untrimmed accuracy of ±3.5% if R1 and R2 are
1% resistors. Other output voltages can also be select-
ed by changing the feedback resistors. Capacitor C
X
sets the oscillator frequency (47pF = 40kHz), while C1
limits output ripple to about 50mV.
With a low-cost molded inductor, the circuit’s efficiency
is about 75%, but an inductor with lower series resis-
tance such as the Dale TE3Q4TA increases efficiency
to around 85%. A key to high efficiency is that the
MAX630 itself is powered from the +15V output. This
provides the onboard N-channel output device with 15V
gate drive, lowering its on-resistance to about 4Ω.
When +5V power is first applied, current flows through
L1 and D1, supplying the MAX630 with 4.4V for startup.
+5V to ±15V DC-DC Converter
The circuit in Figure 3 is similar to that of Figure 1
except that two more windings are added to the induc-
tor. The 1408 (14mm x 8mm) pot core specified is an
IEC standard size available from many manufacturers
(see Table 1). The -15V output is semiregulated, typi-
cally varying from -13.6V to -14.4V as the +15V load
current changes from no load to 20mA.
2.5W, 3V to 5V DC-DC Converter
Some systems, although battery powered, need high
currents for short periods, and then shut down to a low-
power state. The extra circuitry of Figure 4 is designed to
meet these high-current needs. Operating in the buck-
boost or flyback mode, the circuit converts -3V to +5V.
The left side of Figure 4 is similar to Figure 1 and sup-
plies 15V for the gate drive of the external power MOS-
FET. This 15V gate drive ensures that the external device
is completely turned on and has low on-resistance.
The right side of Figure 4 is a -3V to +5V buck-boost
converter. This circuit has the advantage that when the
MAX630 is turned off, the output voltage falls to 0V,
unlike the standard boost circuit, where the output volt-
age is V
BATT
- 0.6V when the converter is shut down.
When shut down, this circuit uses less than 10µA, with
most of the current being the leakage current of the
power MOSFET.
The inductor and output-filter capacitor values have
been selected to accommodate the increased power
levels. With the values indicated, this circuit can supply
up to 500mA at 5V, with 85% efficiency. Since the left
side of the circuit powers only the right-hand MAX630,
the circuit starts up with battery voltages as low as
1.5V, independent of the loading on the +5V output.
MAX630/MAX4193
CMOS Micropower Step-Up
Switching Regulator
_______________________________________________________________________________________ 9
MAX630
1MΩ
95.3kΩ
47pF
7
V
FB
2
C
X
GND LBD
N.C.
GND
1
L
X
3
65
48
I
C
+V
S
+5V
330μF
25V
330μF
25V
1:3:3
220μH PRIMARY
14 x 8mm POT CORE
ALL DIODES IN4148
Figure 3. +5V to ±15V Converter
P1-P3P4-P6P7-P9P10-P12P13-P14

MAX4193ESA+

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Buy MAX4193ESA+
Manufacturer:
Maxim Integrated
Description:
Switching Voltage Regulators CMOS uPower Step Up Switching Reg
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