Applications information – Rainbow Electronics MAX1821 User Manual
Page 13
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
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13
Applications Information
Setting the Output Voltage (MAX1820)
The MAX1820 is optimized for highest system efficiency
when applying power to a linear PA in WCDMA hand-
sets. When transmitting at less than full power, the sup-
ply voltage to the PA is reduced (from 3.4V to as low as
0.4V) to greatly reduce battery current. Figure 4 shows
the typical WCDMA PA load profile. The use of a DC-
DC converter such as the MAX1820 dramatically
reduces battery drain in these applications.
The MAX1820’s output voltage is dynamically
adjustable from 0.4V to V
BATT
by the use of the REF
input. The gain from V
REF
to V
OUT
is internally set to
1.76. V
OUT
can be adjusted during operation by driving
REF with an external DAC. The MAX1820 output
responds to full-scale change in voltage and current in
<30µs.
Setting the Output Voltage (MAX1821)
The MAX1821 is intended for general-purpose step-
down applications where high efficiency is a priority.
Select an output voltage between 1.25V and V
BATT
by
connecting FB to a resistive divider between the output
and GND (Figure 3). Select feedback resistor R2 in the
5k
Ω to 30kΩ range. R1 is then given by:
where V
FB
= 1.25V.
Compensation and Stability
The MAX1820/MAX1821 are externally compensated
by placing a resistor and a capacitor (R
C
and C1) in
series, from COMP to GND (Figure 3). The capacitor
integrates the current from the transimpedance amplifi-
er, averaging output capacitor ripple. This sets the
device speed for transient responses and allows the
use of small ceramic output capacitors because the
phase-shifted capacitor ripple does not disturb the cur-
rent regulation loop. The resistor sets the proportional
gain of the output error voltage by a factor g
m
✕
R
C
.
Increasing this resistor also increases the sensitivity of
the control loop to the output capacitor ripple.
This resistor and capacitor set a compensation zero
that defines the system’s transient response. The load
pole is a dynamic pole, shifting the pole frequency with
changes in load. As the load decreases, the pole fre-
quency shifts to the left. System stability requires that
the compensation zero must be placed properly to
ensure adequate phase margin (at least 30° at unity
gain). The following is a design procedure for the com-
pensation network:
1) Select an appropriate converter bandwidth (f
C
) to
stabilize the system while maximizing transient
response. This bandwidth should not exceed 1/5 of
the switching frequency. Use 100kHz as a reason-
able starting point.
2) Calculate the compensation capacitor, C1, based
on this bandwidth:
Resistors R1 and R2 are internal to the MAX1820; use
R1 = 151k
Ω and R2 = 199kΩ as nominal values for cal-
culations. These resistors are external to the MAX1821
(see the Setting the Output Voltage section). Using
V
OMAX
= 3.4V, I
OMAX
= 0.6A, g
m
= 50µs, R
CS
= 0.75
Ω,
C1 is evaluated as:
TION 3
Selecting the nearest standard value of 330pF corre-
sponds to a 103kHz bandwidth, which is still accept-
able per the above criteria.
C1
3.4V
0.6A
1
0
50 s
1
151k +199k
1
2 3
= 341pF
=
Ч
Ч
Ч
Ч
.
.
75
99
14 100
Ω
Ω
Ω
Ω
µ
k
kHz
C1
V
I
1
R
g
R2
R1+R2
1
2
O(MAX)
O(MAX)
CS
m
C
=
Ч
Ч Ч
π f
R1
V
V
-
OUT
FB
=
R2
1
30
600
WCDMA PA SUPPLY CURRENT (mA)
300
0.4
1.0
3.0
3.4
WCDMA PA SUPPLY VOLTAGE (V)
Figure 4. Typical WCDMA PA Load Profile