Datasheet

Low-Dropout Regulator (LDO)
All MAX1856 functions, including EXT, are internally
powered from the on-chip, low-dropout 5V regulator.
The regulator input is at V
CC
, while its output is at LDO.
The V
CC
-to-LDO dropout voltage is typically 200mV
(300mV max at 12mA), so that when V
CC
is <5.2V,
V
LDO
is typically V
CC
- 200mV. When the LDO is in
dropout, the MAX1856 still operates with V
CC
as low as
3V (as long as the LDO exceeds 2.7V), but with
reduced amplitude FET drive at EXT. The maximum
V
CC
input voltage is 28V.
LDO can supply up to 12mA to power the IC, supply
gate charge through EXT to the external FET, and sup-
ply small external loads. When driving particularly large
FETs at high switching rates, little or no LDO current
may be available for external loads. For example, when
switched at 500kHz, a large FET with 20nC gate charge
requires 20nC
✕
500kHz, or 10mA.
Soft-Start
The MAX1856 features a “digital” soft-start that is pre-
set and requires no external capacitor. Upon startup,
the peak inductor current increments from 1/5th of the
value set by R
CS
, to the full current-limit value in five
steps over 1024 cycles of f
OSC
or f
SYNC
. Additionally,
the oscillator runs at 1/3 the normal operating frequen-
cy (f
OSC
/3) until the output voltage reaches 20% of its
nominal value (V
FB
≤ 1.0V). See the Typical Operating
Characteristics for a scope picture of the soft-start
operation. Soft-start is implemented: 1) when power is
first applied to the IC, 2) when exiting shutdown with
power already applied, and 3) when exiting undervolt-
age lockout. The MAX1856’s soft-start sequence does
not start until V
LDO
reaches 2.5V.
Design Procedure
The MAX1856 can operate within a wide input voltage
range from 3V to 28V. This allows it to be used with wall
adapters. In applications driven by low-power, low-cost
and low input and output ripple current requirements,
the MAX1856 flyback topology can be used to gener-
ate various levels of output voltages and multiple out-
puts.
Communications over the Internet interface with a stan-
dard telephone connection, which includes the
Subscriber Line Interface Circuit (SLIC). The SLIC
requires a negative power supply for the audio and
ringer functions. The circuits discussed here are
designed for these applications. The following design
discussions are related to the standard application cir-
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
_______________________________________________________________________________________ 9
CS
EXT
1
2
FB
LDO
FREQ
*INPUT
4.5V TO 24V
D1, D2: Central Semiconductor CMR1U-02
M1: International Rectifier IRLL2705
T1: Coiltronics CTX01-14853
R
OSC
200kΩ
C4
1µF
C
LDO
1µF
R
CS
33mΩ
C
IN
(2x) 10µF
25V
GND
REF
V
CC
PGND
SYNC/SHDN
MAX1856
OUT2
-72V
M1
9
10
1
2
3
7
4
5
6
8
T1
D2
D1
C5
1nF
R6
100Ω
R5
10Ω
R4
470Ω
R3
5.11kΩ
R2
681kΩ
R1
174kΩ
C
REF
2.2µF
C2
100µF
Sanyo
100MV100AX
C1
330µF
Sanyo
35MV330AX
C3
100pF
2
2
OUT1
-24V
C
FB
1nF
*INPUT RANGE LIMITED BY OUTPUT POWER REQUIREMENTS.
SEE MAXIMUM OUTPUT POWER AND TYPICAL OPERATING CHARACTERISTICS.
Figure 1. Standard Application Circuit