Rainbow Electronics MAX1821 - Manual (Page 12)

MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
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Forced PWM operation uses higher supply current with
no load (3.3mA typ) compared to skip mode.
100% Duty-Cycle Operation
The on-time can exceed one internal oscillator cycle,
which permits operation up to 100% duty cycle. As the
input voltage drops, the duty cycle increases until the
P-channel MOSFET is held on continuously. Dropout
voltage in 100% duty cycle is the output current multi-
plied by the on-resistance of the internal switch and
inductor, approximately 150mV (I
OUT = 600mA). Near
dropout, the on-time may exceed one PWM clock
cycle; therefore, small-amplitude subharmonic ripple
may occur.
COMP Clamp
The MAX1820/MAX1821 compensation network has a
0.45V to 2.15V error regulation range. The clamp pre-
vents COMP from rising too high or falling too low to
optimize transient response.
Dropout
Dropout occurs when the input voltage is less than the
desired output voltage plus the IR drops in the circuit
components. The duty cycle is 100% during this condi-
tion, and the main switch remains on, continuously
delivering current to the output up to the current limit.
IR drops in the circuit are primarily caused by the on-
resistance of the main switch and the resistance in the
inductor.
During dropout, the high-side P-channel MOSFET turns
on, and the controller enters a low-current consumption
mode. Every 6µs (6 cycles), the MAX1820/MAX1821
check to see if the device is still in dropout. The device
remains in this mode until the MAX1820/MAX1821 are
no longer in dropout.
Undervoltage Lockout (UVLO)
The MAX1820/MAX1821 do not operate with battery
voltages below the UVLO threshold of 2.35V (typ). The
BATT input remains high impedance until the supply
voltage exceeds the UVLO threshold. This guarantees
the integrity of the output voltage regulation and pre-
vents excessive current during startup and as the bat-
tery supply voltage drops during usage.
Synchronous Rectification
An N-channel synchronous rectifier eliminates the need
for an external Schottky diode and improves efficiency.
The synchronous rectifier turns on during the second
half of each cycle (off-time). During this time, the volt-
age across the inductor is reversed, and the inductor
current falls. In normal mode, the synchronous rectifier
is turned off when either the output falls out of regula-
tion (and another on-time begins) or when the inductor
current approaches zero. In forced PWM mode, the
synchronous rectifier remains active until the beginning
of a new cycle.
SYNC Input and Frequency Control
The MAX1820Z and MAX1821 internal oscillator is set
to a fixed 1MHz switching frequency. The MAX1820Z
and MAX1821 do not have synchronizing capability
and the SYNC pin must be connected to GND. The
MAX1820Y, MAX1820X, and MAX1821X are capable of
synchronizing to external signals. For external synchro-
nization, drive the SYNC pin with a 13MHz (MAX1820X
and MAX1821X) or 19.8MHz (MAX1820Y) AC-coupled
sine wave. SYNC has a perfect 13:1 (MAX1820X and
MAX1821X) or 18:1 (MAX1820Y) clock divider for 1MHz
(MAX1820X and MAX1821X) or 1.1MHz (MAX1820Y)
switching from common system clocks. The input fre-
quency range for SYNC is 10MHz to 16MHz
(MAX1820X, MAX1821X) or 15MHz to 21MHz
(MAX1820Y). Connect SYNC to GND to use the internal
free-running oscillator at 1MHz.
Shutdown Mode
Drive SHDN to GND to place the MAX1820/MAX1821 in
shutdown mode. In shutdown, the reference, control
circuitry, internal switching MOSFET, and the synchro-
nous rectifier turn off, reducing the supply current to
0.1µA, and the output goes high impedance. Connect
SHDN to BATT for normal operation.
Current-Sense Comparators
The MAX1820/MAX1821 use several internal current-
sense comparators. In PWM operation, the PWM com-
parator terminates the cycle-by-cycle on-time (Figures
1 and 2) and provides improved load and line
response. This allows tighter specification of the induc-
tor-saturation current limit to reduce inductor cost. A
second current-sense comparator used across the P-
channel switch controls entry into skip mode. A third
current-sense comparator monitors current through the
internal N-channel MOSFET to prevent excessive
reverse currents and determine when to turn off the
synchronous rectifier. A fourth comparator used at the
P-channel MOSFET detects overcurrent. This protects
the system, external components, and internal
MOSFETs under overload conditions.