MAX1775 Maxim, MAX1775 Datasheet - Page 10

• Manufacturer Part Number: MAX1775
• Description: Dual-Output Step-Down DC-DC Converter for PDA/Palmtop Computers
• Manufacturer: Maxim

Dual-Output Step-Down
DC-DC Converter for PDA/Palmtop Computers
Figure 1. Typical Application Circuit (Low Input Voltage)
The MAX1775 dual step-down DC-DC converter is
designed to power PDA, palmtop, and subnotebook
computers. Normally, these devices need two separate
power supplies—one for the processor and another
higher voltage supply for the peripheral circuitry. The
MAX1775 provides an adjustable +1.25V to +5.5V main
output designed to power the peripheral circuitry of
PDAs and similar devices. The main output delivers
over 2A output current. The lower voltage core convert-
er has an adjustable +1.0V to +5.0V output, providing
up to 1.5A output current. Both regulators utilize a pro-
prietary regulation scheme, allowing PWM operation at
medium to heavy loads, and automatically switch to
pulse skipping at light loads for improved efficiency.
Figure 1 is the typical application circuit.
When delivering low output currents, the MAX1775
operates in discontinuous conduction mode. Current
through the inductor starts at zero, rises above the min-
imum current limit, then ramps down to zero during
each cycle (see Typical Operating Characteristics).
The switch waveform may exhibit ringing, which occurs
at the resonant frequency of the inductor and stray
10
______________________________________________________________________________________
Detailed Description
Operating Modes for the
0.22µF
ON
ON
1µF
Step-Down Converters
C2
C3
OFF
OFF
14
1
2
5
9
3
CVL
REF
PGND
GND
SHDNM
SHDNC
MAX1775
2.7V TO 5.5V
6
IN
IN
CVH
PDRV
NDRV
FBM
CS+
LXC
FBC
8
INC
CS-
C1
1µF
15
7
4
11
12
10
16
13
M1
M2
5.4µH
L
C
R4
R5
L
5µH
M
capacitance, due to the residual energy trapped in the
core when the rectifier MOSFET turns off. This does not
degrade the circuit performance.
When delivering medium-to-high output currents, the
MAX1775 operates in PWM continuous-conduction
mode. In this mode, current always flows through the
inductor and never ramps to zero. The control circuit
adjusts the switch duty cycle to maintain regulation
without exceeding the peak switching current set by
the current-sense resistor.
The MAX1775 operates with a duty cycle up to 100%.
This feature extends the input voltage range by turning
the MOSFET on continuously when the supply voltage
approaches the output voltage. This services the load
when conventional switching regulators with less than
100% duty cycle would fail. Dropout voltage is defined
as the difference between the input and output volt-
ages when the input is low enough for the output to
drop out of regulation. Dropout depends on the MOS-
FET drain-to-source on-resistance, current-sense resis-
tor, and inductor series resistance, and is proportional
to the load current:
C6
10µF
I
OUT
33mΩ
C5
22µF
R1
CORE
1.8V
1.5A
✕
[R
DS(ON)
100% Duty Cycle and Dropout
Dropout voltage =
R2
R3
+ R
MAIN
3.3V
2A
C4
47µF
SENSE
+ R
INDUCTOR
]