MAX1856 Maxim, MAX1856 Datasheet - Page 9

• Manufacturer Part Number: MAX1856
• Description: Wide Input Range / Synchronizable / PWM SLIC Power Supply
• Manufacturer: Maxim

Figure 1. Standard Application Circuit
All MAX1856 functions, including EXT, are internally
powered from the on-chip, low-dropout 5V regulator.
The regulator input is at V
The V
(300mV max at 12mA), so that when V
V
dropout, the MAX1856 still operates with V
3V (as long as the LDO exceeds 2.7V), but with
reduced amplitude FET drive at EXT. The maximum
V
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
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
steps over 1024 cycles of f
LDO
CC
input voltage is 28V.
CC
is typically V
-to-LDO dropout voltage is typically 200mV
4.5V TO 24V
*INPUT RANGE LIMITED BY OUTPUT POWER REQUIREMENTS.
SEE MAXIMUM OUTPUT POWER AND TYPICAL OPERATING CHARACTERISTICS.
✕
*INPUT
CS
500kHz, or 10mA.
Low-Dropout Regulator (LDO)
, to the full current-limit value in five
_______________________________________________________________________________________
CC
200kΩ
C
- 200mV. When the LDO is in
R
1µF
1µF
10Ω
LDO
C4
OSC
R5
CC
OSC
, while its output is at LDO.
10
Wide Input Range, Synchronizable,
9
2
1
3
7
or f
V
SYNC/SHDN
LDO
FREQ
GND
PGND
CC
SYNC
MAX1856
. Additionally,
CC
CC
Soft-Start
is <5.2V,
as low as
(2x) 10µF
EXT
REF
CS
FB
25V
C
8
6
5
4
IN
C5
1nF
R3
5.11kΩ
C
2.2µF
100Ω
REF
R6
PWM SLIC Power Supply
the oscillator runs at 1/3 the normal operating frequen-
cy (f
nominal value (V
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
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-
1
M1
R
33mΩ
OSC
C
1nF
CS
FB
D1, D2: Central Semiconductor CMR1U-02
M1: International Rectifier IRLL2705
T1: Coiltronics CTX01-14853
T1
/3) until the output voltage reaches 20% of its
2
2
2
LDO
681kΩ
D1
FB
R2
reaches 2.5V.
D2
≤ 1.0V). See the Typical Operating
R4
470Ω
C3
100pF
R1
174kΩ
Design Procedure
OUT2
-72V
C2
100µF
Sanyo
100MV100AX
C1
330µF
Sanyo
35MV330AX
OUT1
-24V
9