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19-1287; Rev 0; 9/97
MAX774 ISDN, Ring-Tone,
Power-Supply Evaluation Kit
_______________General Description
The MAX774 ISDN ring-tone power-supply (IRG) evalua-
tion kit (EV kit) provides the high voltages required for
implementing a plain old telephone system (POTS) inter-
face on ISDN modems and line cards. It is a fully assem-
bled and tested board that provides a tightly regulated,
-24V output for powering off-hook voice communication
and a -70V output for on-hook, ring-tone generation.
The EV kit is designed for applications that implement
the telephone interface using subscriber line interface
circuit (SLIC) ICs, such as the AM79R79 from AMD and
comparable products from Lucent, Harris, and other
vendors. Its design feeds back the -24V output, achiev-
ing tight regulation for clean voice-signal transmission.
An economical, off-the-shelf, surface-mount transformer
reduces system cost and size. Compact design con-
serves board area. High efficiency and reduced quies-
cent current make this design the optimal solution for
green PC and portable designs.
The MAX774 IRG EV kit can also be used to evaluate
the MAX775/MAX776. It has a layout that allows modifi-
cation for -48V output operation as well as adaptation
for lower-voltage European applications.
Caution: Touching the MAX774 IRG EV kit’s -70V
output can result in electrical shock. Do not touch
the -70V output during operation or for five minutes
after operation. Maxim assumes no liability for
injury or damage resulting from unsafe operation of
this EV kit.
____________________________Features
o
+3V to +16.5V Operating Range
o
Tightly Regulated, -24V Output for Off-Hook
Voice Communication
o
-70V Output Supports a Five-Ringer-Equivalent
Load (V
IN
> 10.5V)
o
Compact Construction
o
Proven PC Board Design
o
Uses Off-the-Shelf Components
o
Up to 84% Efficiency
o
5µA Shutdown Current
o
Fully Assembled and Tested
Evaluates: MAX774/MAX775/MAX776
______________Ordering Information
PART
MAX774IRGEVKIT
TEMP. RANGE
0°C to +70°C
BOARD TYPE
Mixed Surface Mount
and Through-Hole
_____________________________________________________________Component List
DESIGNATION QTY
C1
C2
C3, C7
C4
C5, C6
C8
C9
1
1
2
1
2
1
1
DESCRIPTION
100pF, 100V ceramic capacitor
1nF, 50V ceramic capacitor
0.1µF, 50V ceramic capacitors
0.33µF, 25V ceramic capacitor
68µF, 20V, low-ESR tantalum capacitors
AVX TPSE686M020R0150
0.1µF, 100V ceramic capacitor
220µF, 35V, low-ESR
aluminum-electrolytic capacitor
Sanyo 35CV220GX
120µF, 63V, low-ESR
aluminum-electrolytic capacitor
Sanyo 63MV120GX
1A, 100V Schottky diode
Motorola MBRS1100T3
P1
R1
R2
R3
R4
R5, R6
U1
1
1
1
1
1
2
1
L1
1
DESIGNATION QTY
D2
1
DESCRIPTION
1A, 200V, ultra-fast diode
Nihon EC11FS2
10µH, 3.2A transformer
Coiltronics VP2-0216
60V, R
DS(ON)
= 0.15Ω
P-MOSFET (D-PAK)
Motorola MTD20P06HDL
1MΩ, 1% resistor
63.4kΩ, 1% resistor
68mΩ, 1/2W, metal-strip resistor
Dale WSL-2010-R068-F
330kΩ, 100V, 5% resistor
8.2kΩ, 5% resistors
Inverting controller IC (8 SO)
Maxim MAX774CSA
1
C10
1
D1
1
________________________________________________________________
Maxim Integrated Products
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
MAX774 ISDN, Ring-Tone,
Power-Supply Evaluation Kit
Evaluates: MAX774/MAX775/MAX776
______________Component Suppliers
SUPPLIER
AVX
Coiltronics
Dale-Vishay
IRC
Motorola
Nichicon
Nihon
Raychem
Sanyo
Sprague
Vishay/Vitramon
PHONE
(803) 946-0690
(561) 241-7876
(402) 564-3131
(512) 992-7900
(602) 303-5454
(847) 843-7500
(805) 867-2555
(650) 361-6900
(619) 661-6835
(603) 224-1961
(203) 268-6261
FAX
(803) 626-3123
(561) 241-9339
(402) 563-6418
(512) 992-3377
(602) 994-6430
(847) 843-2798
(805) 867-2698
(650) 361-5575
(619) 661-1055
(603) 224-1430
(203) 452-5670
tightly regulated voltage around -24V or -48V for off-
hook signal transmission. The second is a loosely regu-
lated -70V for ring-tone generation. Servicing a typical
five-ringer equivalent load requires a current around
100mA or more from the -70V supply, depending on
the SLIC IC and the ring-generation scheme.
The MAX774 IRG EV kit can service a SLIC with a five-
phone ringer equivalent load (approximately 9W) from
a 12V ±10% input. It operates down to 3V, and pro-
vides 2.4W from 3.3V and 3.9W from 5V. Use of an
inexpensive off-the-shelf transformer, such as the
Versa-Pac™ model VP2-0216, provides both high-volt-
age outputs from a single inverting DC-DC controller,
reducing board area and component costs. Selection
of a transformer with multifilar winding enhances cross
regulation by improving voltage coupling between the
outputs and reducing spiking from leakage inductance.
The two outputs are implemented by connecting three
pairs of transformer windings in series. The -24V output
is obtained by connecting a diode (D1) and output filter
capacitor (C9) to the first pair of windings. Feeding
back this output achieves tight regulation. The -70V
output is derived from the third pair of windings. Loose
regulation of this output is obtained by the turns ratio
with the -24V output.
_____________________________________Quick Start
The MAX774 IRG evaluation kit (EV kit) is fully assem-
bled and tested. Follow these steps to verify board
operation.
Do not turn on the power supply until all
connections are completed.
1) Connect a 12V, 2A power-supply ground terminal to
a GND pad on the MAX774 IRG EV kit.
2) Monitor the input current by connecting the power
supply's positive terminal to the EV kit’s VIN input
through a current meter.
3) Attach a voltmeter across the EV kit’s VIN and GND
inputs to monitor input voltage.
4) Connect voltmeters to each of the EV kit’s outputs
labeled -70V and -24V.
5) Connect the SHDN pad to GND.
6) Turn on the power supply and slowly increase the
voltage to 12V.
7) Monitor the outputs for correct voltage and check
the input for typical supply current (20mA at 12V).
Circuit Operation
The EV kit schematic (Figure 1) and the MAX774 block
diagram in the MAX774/MAX775/MAX776 data sheet
show how the circuit works. When the -24V output
drops out of regulation, the error comparator in the
MAX774 initiates a switching cycle. The P-channel
MOSFET (P1) turns on, allowing current to ramp up
through the transformer’s lower windings (between the
1/3 tap and ground) and store energy in a magnetic
field. When the current through the sense resistor
crosses the trip threshold (210mV / 68mΩ = 3.09A), the
MOSFET turns off and interrupts the current flow, caus-
ing the magnetic field in the transformer to collapse.
The transformer forces current through the output
diodes, transferring the stored energy to the output fil-
ter capacitors. The output filter capacitors smooth the
power and voltage delivered to the load. The MAX774
waits until it senses the output dropping below the reg-
ulation trip point before initiating another cycle. The
-24V output is precisely regulated by connecting a volt-
age divider, R1 and R2, as shown in Figure 1. The
MAX774 regulates the FB pin, keeping it at 0V. The
-70V output is regulated using the turns ratios between
the -24V and -70V output.
_______________Detailed Description
The MAX774 IRG EV kit provides the high voltages
required for implementing a plain old telephone system
(POTS) interface on ISDN modems and other telephone
line cards. These boards typically employ ICs such as
the AM79R79 Ringing Subscriber Line Interface Circuit
(SLIC) from AMD. These ICs generate an analog tele-
phone interface by providing both off-hook and on-
hook signal transmission, ring-tone generation, and
ring-trip detection. Ringing SLIC ICs typically require
two high-voltage power-supply inputs. The first is a
Versa-Pac is a trademark of Coiltronics Corp.
2
_______________________________________________________________________________________
MAX774 ISDN, Ring-Tone,
Power-Supply Evaluation Kit
Evaluates: MAX774/MAX775/MAX776
INPUT
3V TO 15V, 12V ±10%
FOR FIVE-PHONE LOAD
C5
68µF
20V
C6
68µF
20V
C4
0.33µF
SHDN
REF
V+
R3
68mΩ
CS
U1
MAX774
R2
63.4k
C3
0.1µF
C2
1.0nF
-70V OUTPUT
C1
100pF
100V
-24V OUTPUT
R5
8.2k
R6
8.2k
C7
0.1µF
50V
D1
C9
220µF 100V
35V
R1
1M
R4
330k
C10
120µF
63V
C8
0.1µF
100V
FB
OUT
GND
EXT
P1
D2
200V
2/3 TAP
L1, 10µH, 3A
1/3 TAP
Figure 1. MAX774 IRG EV Kit Schematic
Output Filter Capacitors
The positive pin of the filter capacitor for the -70V out-
put is connected to the -24V output rather than ground
to simplify board layout, enhance stability, allow the use
of a lower-cost lower-voltage capacitor, and improve
cross-regulation. Ripple on the -24V output is about
200mV and can be reduced further using a capacitor
with lower ESR. The Sanyo MV-GX series is recom-
mended.
-70V output. Dale’s LPE6855-100MB and LPE6562-
100MB also work, but have different footprints and
pinouts and require almost double preloading.
If lower output power is desired, increase the current-
sense-resistor value and transformer inductance propor-
tionally. For example, when reducing power capability to
one-half of the current design, double the current-sense
resistor to around 130mΩ and the transformer induc-
tance per winding to around 20µH to 33µH.
__________Applications Information
This section is intended to aid in transferring the EV kit
design to a finished product.
Cross Regulation
The -70V output is derived from the -24V output by
stacking pairs of windings in an autotransformer config-
uration. Cross regulation between the two outputs, how-
ever, has limitations. In the on-hook and ringing case,
when the -24V output is lightly loaded with the -70V out-
put heavily loaded, the -70V output droops. In the off-
hook case with the -24V output heavily loaded and the
-70V output lightly loaded, the -70V output rises. These
effects occur in all transformer-based flyback solutions
when the outputs are dissimilarly loaded.
3
Transformer Selection
Choose a transformer with an inductance around 10µH
to 15µH per winding, with a saturation-current rating
greater than 3A. The MAX774 IRG EV kit uses
Coiltronics’ Versa-Pac model VP2-0216. This economi-
cal, off-the-shelf transformer uses two trifilar windings
for superior coupling and improved regulation of the
_______________________________________________________________________________________
MAX774 ISDN, Ring-Tone,
Power-Supply Evaluation Kit
Evaluates: MAX774/MAX775/MAX776
Preloading
Use preloading at the outputs to keep the -70V output
in regulation. For designs servicing a five-ringer equiva-
lent load, use the following preloads. For the off-hook
case, only a couple hundred microamperes are neces-
sary to hold down the -70V output. This can be
achieved using either a 330kΩ resistor (R4, Figure 1) or
zener diode (Figure 2b). For the on-hook case, draw
approximately 5.5mA from the -24V output to hold up
the -70V output. This 5.5mA can be drawn continuously
using two 8.2kΩ resistors (R5 and R6), or intermittently
using a transistor to gate the preload while the phone is
ringing (Figure 2c). The transistor can be controlled
using a microcontroller input/output line, or it can be
decoded from the control signals of the AM79R79.
To optimize performance or efficiency in applications
servicing a different ringer-equivalent load, use the pre-
loading curves for guidance (Figure 3 and 4). Use
-24V OR -70V OUTPUT
Figure 3 to determine the minimum preloading needed
on the -24V output for adequate regulation of the -70V
output while the SLIC IC is ringing phones (on-hook
case). For example, approximately 50mA is required for
a two-phone load. First, follow the vertical line from the -
70V output axis up to curve A or B. Next, follow the hor-
izontal lines to the corresponding point on the -24V
Output Minimum Load axis, in this case 2.5mA using
curve A. Preload the -24V output with this current using
a resistor R = V / I or 24V / 2.5mA = 9.6kΩ. Round
down to the nearest standard value (9.1kΩ). The power
rating of the resistor must exceed V
2
/ R = 24V
2
/ 9.
1kΩ = 63mW.
Use Figure 4 to determine the preloading needed to
hold down the -70V output when the -24V output is
heavily loaded during off-hook communication. This
preloading is intended to protect the AM79R79. The
VBAT1 pin of this SLIC IC has a -75V operational range
and a -80V absolute maximum rating. If a zener diode
is used for preloading, set the zener voltage rating suf-
ficiently above the regulation set point to prevent
unnecessary current draw.
Efficiency, Quiescent Current,
and Preloading
a) RESISTOR PRELOADING
-70V OUTPUT
MOTOROLA
1SMB5946BT3
75V, 1.5W
ZENER DIODE
b) ZENER CLAMP
The MAX774 is a pulse-frequency-modulation (PFM)
controller designed primarily for use in portable appli-
cations. It improves efficiency and reduces quiescent
current by switching only as needed to service the
load. Prior to preloading, this circuit’s efficiency can be
up to 84%, and quiescent current is around 170µA.
Resistor preloading reduces efficiency and increases
-24V OUTPUT MINIMUM LOAD (mA)
9
8
7
6
5
4
3
2
1
A: PRELOAD FOR -5% REGULATION
B: PRELOAD FOR -10% REGULATION
L1 = VP2 - 0216
-24V OUTPUT
5V LOGIC INPUT FROM
µC
PORT OR DECODING FROM
79R79 CONTROL LINES
R
B
33kΩ FOR 6mA
PRELOAD
R
C
4.7kΩ FOR 6mA
PRELOAD,
2N2907A,
b = 50 min
B
A
5V SUPPLY INPUT
c) SWITCHABLE PRELOADING
0
1
10
100
-70V OUTPUT LOAD CURRENT (mA)
Figure 2. Fixed and Switchable Preloading Schemes
Figure 3. Cross Regulation for -24V Output Preload Selection
(on-hook case)
4
_______________________________________________________________________________________
MAX774IRG EV FIG03
10
MAX774 ISDN, Ring-Tone,
Power-Supply Evaluation Kit
quiescent current. Switchable preloading on the -24V
output (Figure 2c), combined with zener clamping of
the -70V output (Figure 2b) can be used to reduce cir-
cuit current consumption.
Connect the ground terminal of the -70V filter capacitor
to the -24V output rather than to ground. (This also
improves transient response and simplifies layout.)
The MAX774 uses a PFM control scheme that adjusts
the pulse rate to regulate power and voltage to the
load. Pulse spacing decreases with increasing load. As
the pulses begin touching each other, the circuit transi-
tions into continuous-conduction mode. Stable transi-
tion into continuous conduction occurs through pulse
grouping, with gaps less than two cycles wide between
groups, and output ripple no larger than the single-
cycle voltage ripple at light loads (Figure 6).
Poor PC board layout or improper compensation can
cause instability by corrupting the feedback signals.
Instability is identified by either grouped pulses, large
gaps between groups, or output ripple larger than the
single-cycle voltage ripple (Figure 7). It can cause
increased audio interference. Test for instability with a
Evaluates: MAX774/MAX775/MAX776
Current Limiting and Overload Protection
Neither this EV kit nor competing solutions have a prac-
tical level of current protection at the outputs. Use the
current-limiting features built into the AM79R79 SLIC IC
as described in the data sheet for that product. Using
PolySwitch™ resettable fuses at the outputs adds pro-
tection to the system at little expense (Figure 5). With a
PolySwitch, use faster models such as the surface-
mount SMD series.
The MAX774 uses an internal current-sense compara-
tor that provides pulse-by-pulse input current limiting.
However, like competing flyback solutions, this trans-
lates to power (and not current) limiting at the output.
As the output voltage pulls down during overload, the
output current can become high (essentially P
IN(MAX)
/
V
OUT
) until inefficiency and parasitic resistance in the
circuit dominate. Since the circuit is designed for 9W
(min) output to service a five-phone load, short-circuit
currents can reach several amperes.
FROM
SYSTEM
POWER
SUPPLY
Stability and Feedback Compensation
The MAX774 IRG EV kit has been compensated and
tested for a full range of loads. When implementing the
circuit, ensure stability by following the EV kit board
and component list (see
PC Board Layout
section). Use
NPO or COG ceramic capacitors for C1 and C2.
PolySwitch is a trademark of Raychem Corp.
A
A: LOAD FOR +75% REGULATION
B: LOAD FOR +10% REGULATION
MAX774IRG EV FIG04
POLYSWITCH
RING-
GENERATOR
POWER
-70V
SUPPLY
-24V OR
-48V
79R79 OR
COMPARABLE
SLIC CIRCUIT
OUTPUT
TO
PHONE
POLYSWITCH
Figure 5. Overload Protection Using Raychem PolySwitch
Resettable Fuses
1.2
-70V OUTPUT MINIMUM LOAD (mA)
1.0
0.8
0.6
A
0.4
0.2
B
0.0
10
100
-24V OUTPUT LOAD CURRENT (mA)
B
C
5µs/div
V
OUT1
= -23.6V, V
OUT2
= -70V, I
OUT2
= -30mA, V
IN
= 9V
1000
A: MOSFET DRAIN, 20V/div
B: V
OUT1
, 100mV/div, AC COUPLED
C: TRANSFORMER CURRENT, 1A/div
Figure 4. Cross Regulation for -70V Output Preload Selection
(off-hook case)
Figure 6. Normal Light-Load Switching Waveforms
_______________________________________________________________________________________
MAX774IRG EV FIG06
5

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