MAX1702B Datasheet, PDF (13/19 Pages) Maxim Integrated Products – Triple-Output Power-Management IC for Microprocessor-Based Systems

Triple-Output Power-Management IC for
Microprocessor-Based Systems
Functional Diagram
IN
DBI
LBI
LBO
OUTOK
ON2
MAX1702B
REG1
DEAD-
BATTERY
DBO
DETECTOR
LOW-
BATTERY
DETECTOR
ON/OFF
CONTROL
LOGIC
EN
REF
DC-DC BUCK
WITH SKIP
1MHz PWM
REG2
POK
EN
INP1
LX1
PG1
OUT1
COMP1
INP2
LX2
RSO
MR
RESET
TIMER
REF
DC-DC BUCK
WITH SKIP
1MHz PWM
REG3
EN
PG2
FB2
COMP2
INP3
LX3
BANDGAP
REFERENCE
GND
REF
REF
DC-DC BUCK
WITH SKIP
1MHz PWM
PG3
OUT3
COMP3
PGM3
Detailed Description
The MAX1702B triple-output step-down DC-DC convert-
er is ideal for powering PDA, palmtop, and subnotebook
computers. Normally, these devices require separate
power supplies for the processor core, memory, and the
peripheral circuitry. The MAX1702B’s REG1 provides a
fixed 3.3V output designed to power the microprocessor
I/O and other peripheral circuitry. REG1 delivers up to
900mA output current. The microprocessor core is pow-
ered from REG2, which has an adjustable 0.7V to V_IN
output, providing up to 400mA output current. The third
output, REG3, is designed to power memory. REG3 out-
put voltage is set to one of 3 voltages; 3.3V (PGM3 =
IN), 2.5V (PGM3 = REF), or 1.8V (PGM3 = GND) and
delivers up to 800mA of output current. All three regula-
tors utilize a proprietary regulation scheme allowing
PWM operation at medium to heavy loads, and automat-
ically switch to pulse skipping at light loads for
improved efficiency. Under low-battery conditions, the
MAX1702B issues a warning (LBO output).
The MAX1702B employs PWM control at medium and
heavy loads, and skip mode at light loads (below
approximately 80mA) to improve efficiency and reduce
quiescent current to 485µA. During skip operation, the
MAX1702B switches only as needed to service the load,
reducing the switching frequency and associated losses
in the internal switch, the synchronous rectifier, and the
external inductor.
There are three steady-state operating conditions for the
MAX1702B. The device performs in continuous conduc-
tion for heavy loads. The inductor current becomes dis-
continuous at light loads, requiring the synchronous
rectifier to be turned off before the end of a cycle as the
inductor current reaches zero. The device enters into
skip mode when the converter output voltage exceeds
its regulation limit before the inductor current reaches
the pulse-skip threshold.
During skip mode, a switching cycle initiates when the
output voltage drops below the regulation voltage. The
P-channel MOSFET switch turns on and conducts cur-
rent to the output-filter capacitor and load until the
inductor current reaches the pulse-skip current thresh-
old. Then the main switch turns off, and the current flows
through the synchronous rectifier to the output-filter
capacitor and the load. The synchronous rectifier is
turned off when the inductor current approaches zero.
The MAX1702B waits until the output voltage drops below
the regulation voltage again to initiate the next cycle.
100% Duty-Cycle Operation
If the inductor current does not rise sufficiently to sup-
ply the load during the on-time, the switch remains on,
allowing operation up to 100% duty cycle. This allows
the output voltage to maintain regulation while the input
voltage approaches the regulation voltage. Dropout
voltage is the output current multiplied by the on-resis-
tance of the internal switch and inductor, approximately
220mV for an 800mA load for REG1 and REG3 and
150mV for a 400mA load on REG2.
Near dropout, the on-time may exceed one PWM clock
cycle; therefore, small amplitude subharmonic ripple
can occur in the output voltage. During dropout, the
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