MAX1820XEUB+T Maxim Integrated Products, MAX1820XEUB+T Datasheet - Page 11

• Manufacturer Part Number: MAX1820XEUB+T
• Description: IC REG BUCK WCDMA 10-MSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1820XEUB+T

• Applications: Converter, WCDMA Power Amplifier Applications
• Voltage - Input: 2.6 ~ 5.5 V
• Number Of Outputs: 1
• Voltage - Output: 0.4 ~ 3.4 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

the load. The duty cycle of a buck step-down converter
is ideally a ratio of the output voltage to input voltage in
steady-state condition.
The MAX1820/MAX1821 have internal switch current
limits of 1.2A (typ). If I
high-side FET turns off and the synchronous rectifier
turns on. This lowers the duty cycle and causes the out-
put voltage to droop as long as the load current
remains excessive. There is also a synchronous rectifier
current limit of -0.85A when the device is operating in
forced PWM mode (see the Forced PWM Operation sec-
tion). If the negative current limit is exceeded, the syn-
chronus rectifier is turned off, and the inductor current
continues to flow through its body diode until the begin-
ning of the next cycle or the inductor current drops to
zero. This means there is a limit on how much current
the device is allowed to shuttle in response to output
power reduction.
Connecting SKIP to GND enables MAX1820/MAX1821
normal operation (Figure 3). This allows automatic PWM
control at medium and heavy loads and skip mode at
light loads to improve efficiency and reduce quiescent
current to 180µA. Operating in normal mode also allows
the MAX1820/MAX1821 to pulse skip when the peak
inductor current drops below 130mA, corresponding to
a load current of approximately 65mA.
During skip operation, the MAX1820/MAX1821 switch
only as needed to service the load, reducing the
switching frequency and associated losses in the inter-
nal switch, the synchronous rectifier, and the external
inductor.
Figure 3. Standard Operating Circuit
______________________________________________________________________________________
LX
Normal Mode Operation
* CAN BE OMITTED IF CERAMIC OUTPUT CAPACITOR IS USED.
exceeds this maximum, the
10 F
V
IN
= 2.6V TO 5.5V
0.047 F
0.1 F
WCDMA Cellular Phone 600mA
SYNC
REF
BATT
SHDN
SKIP
MAX1821
GND
COMP
PGND
LX
FB
There are three steady-state operating conditions for
the MAX1820/MAX1821 in normal mode. The device
performs in continuous conduction for heavy loads in a
manner identical to forced PWM mode. The inductor
current becomes discontinuous at medium loads,
requiring the synchronous rectifier to be turned off
before the end of a cycle as the inductor current reach-
es zero. The device enters into skip mode when the
converter output voltage exceeds its regulation limit
before the inductor current reaches its skip thres-
hold level.
During skip mode, a switching cycle initiates when the
output voltage has dropped out of regulation. The P-
channel MOSFET switch turns on and conducts current
to the output-filter capacitor and load until the inductor
current reaches the skip peak current limit. Then the
main switch turns off, and the magnetic field in the
inductor collapses, while current flows through the syn-
chronous rectifier to the output filter capacitor and the
load. The synchronous rectifier is turned off when the
inductor current reaches zero. The MAX1820/ MAX1821
wait until the skip comparator senses a low output volt-
age again.
Connect SKIP to BATT for forced PWM operation.
Forced PWM operation is desirable in sensitive RF and
data-acquisition applications to ensure that switching
harmonics do not interfere with sensitive IF and data-
sampling frequencies. A minimum load is not required
during forced PWM operation since the synchronous
rectifier passes reverse-inductor current as needed to
allow constant-frequency operation with no load.
4.7 H
R
82k
C1
330pF
C
4.7 F
Buck Regulators
C2*
1pF
V
OUT
R1
6k
R2
30k
= 1.5V
Forced PWM Operation
11