MAX1688EUE Maxim Integrated Products, MAX1688EUE Datasheet - Page 10

• Manufacturer Part Number: MAX1688EUE
• Description: IC CONV DCDC LIMIT GSM 16-TSSOP
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX1688EUE

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 6 V
• Current - Output: 2.5A
• Frequency - Switching: 1MHz
• Voltage - Input: 2.7 ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP
• Power - Output: 457mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
The value of the output capacitor determines the
amount of power available to deliver to the power
amplifier during the RF burst. A larger output capacitor
with low ESR reduces the amount of output voltage
droop during an RF burst. Use the following equation to
determine capacitor size when ON is synchronized to
the RF burst:
where C
power amplifier burst current, t
period, D
erance, and V
put during the current burst.
For example, when used in a typical GSM system,
t
(1.42A for a +5.5V system), and with a droop of less
than 10%, the value of the capacitor is 5.3mF ±20%.
The output capacitor also determines the constant-load
(ON connected to V
ple is:
where I
10
Figure 6a. MAX1687 Typical Application Circuit (GSM Pulsed Load)
GSM
C
OUT
______________________________________________________________________________________
V
= 4.62ms, I
=
RIPPLE
RIPPLE
OUT
GSM
V
DROOP
is the output capacitor, I
= I
DROOP
is typically 0.2A.
is the duty cycle, tol is the capacitor tol-
RIPPLE
- I
Output (Reservoir) Capacitor
BURST
2.7V TO 6V
BURST
CC
is the acceptable drop in the out-
D
V
GSM
) ripple voltage. The output rip-
IN
· ESR
0.1 F
= 2.66A for a +3.6V system
OUTPUT
0 TO 1V
ESR
DAC
(OUTPUT CAPACITOR)
I
BURST
OFF
OUTPUT CAPACITOR
GSM
is the current pulse
t
BURST
GSM
47 F
ON
is the peak
LX1
LX1
IN
IN
ON
LIM
REF
1 - tol
MAX1687
10 H
The current limit of the MAX1687 can be set by an exter-
nal DAC (Figure 6a), making it variable by using a micro-
controller. The MAX1687 is the choice for systems
interfacing with a microcontroller, but may also be used
with fixed current limit (Figure 6b). The MAX1688 can
monitor the droop of the output voltage to set the current
limit, maximizing battery life. The MAX1688 is suitable for
systems demanding variable burst currents (Figures 6a,
6b, and 7) as well as variable input voltages.
The MAX1687/MAX1688’s high-frequency operation
and high peak currents make PC board layout critical
to minimize ground bounce and noise. Locate input
bypass and output filter capacitors as close to the
device pins as possible. All connections to OUT and FB
should also be kept as short as possible. Use a low-
inductance ground plane. Connect the ground leads of
the input capacitor, output capacitor, and PGND pins in
a star configuration to the ground plane. Table 1 lists
suggested suppliers. Refer to the MAX1687/MAX1688
evaluation kit manual for a suggested surface-mount
layout and a list of suggested components.
AGND
PGND
PGND
OUT
OUT
LX2
LX2
FB
Typical Application Circuits
R2
187k
R1
61.9k
2000 F
V
2A AT 12%
DUTY CYCLE
OUT
= 5V
Layout