MAX1687EUE+T Maxim Integrated Products, MAX1687EUE+T Datasheet - Page 9

• Manufacturer Part Number: MAX1687EUE+T
• Description: IC DC-DC CONV STP UP 16TSSOP
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX1687EUE+T

• 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: Lead free / RoHS Compliant

To set the current limit, apply a voltage of 0 to 1V at
LIM. The current limit is 200mA when V
0.25V. Use the following equation to calculate I
where V
V
applied at V
of three methods: an externally applied voltage, the
output of a DAC, or a resistor-divider using V
supply voltage (TSSOP packages) (Figure 5). Note that
REF can supply up to 10µA.
Determine V
where I
the Inductor Current parameter in the Typical Operating
Characteristics ).
The MAX1688 has a recharging feature employing a
sample-and-hold, which sets the maximum time to
recharge the reservoir capacitor. Synchronize the ON
pin to place the converter in standby during each load
current burst. At the end of each load current burst, the
output voltage is sampled by the MAX1688. This volt-
age controls the peak inductor current. The greater the
difference between the regulated output voltage and
the valley of the sag voltage, the higher the peak cur-
rent. This results in a constant recharge time that com-
pensates for varying output filter capacitor character-
istics as well as a varying input voltage. Therefore, the
circuit demands only as much peak current from the
battery as output conditions require, minimizing the
peak current from the battery. An external resistor
Figure 5. Current-Limit Adjust
LIM
REF
is internally clamped to 1.25V when the voltage
a)
LX(PEAK)
LIM
LIM
Setting Recharge Time (MAX1688)
V
= 0.25V to 1V.
LIM
LIM
MAX1687
LIM
I
LIM
c)
as follows:
is above 1.25V. Generate V
= (I
= [(I
= V
_______________________________________________________________________________________
Precise, Adaptive Current Limit for GSM
R3
R4
LX(PEAK)
LIM
LOAD
REF
LIM
(0.86A/V) – 0.06A
MAX1687
· V
DAC
+ 0.06A) / 0.86
OUT
b)
) / V
LIM
V
R3 + R4 > 125k
LIM(CHG)
IN
MAX1687
Step-Up DC-DC Converters with
] + 0.1A (see
= V
LIM
LIM
REF
REF
LIM
R4 + R3
by one
= 0 to
R4
as the
:
between CHG and GND controls the output recharge
time. A large resistor increases peak inductor current
which speeds up recovery time. Calculate the resistor
as follows:
where:
For example, for I
= +2.7V, and V
a 5% tolerance resistor.
The recovery time for a 40.2kΩ R
that with an 18kΩ R
higher. See Switching Waveforms (GSM Pulsed Load
1A, R
Pulsed Load 1A, R
Characteristics .
The value of the inductor determines the switching fre-
quency. Calculate the switching frequency as:
where f is the switching frequency, V
age, V
and I
0.2A. Using a lower value inductor increases the fre-
quency and reduces the physical size of the inductor.
A typical frequency is from 150kHz to 1MHz (see
Switching Frequency vs. Inductance in the Typical
Operating Characteristics ).
R
I
D
V
V
V
V
rent burst
g
g
tol is the tolerance of the R
BURST
R
IN
OUT
REF
DROOP
mCHG
mFB
CHG
GSM
CHG
is the input voltage
RIPPLE
CHG
OUT
= 1.25V
is the output voltage
is the feedback transconductance = 200µA/V
is the external resistor
is the duty cycle of GSM
=
f = V
is the peak burst current expected
is the internal transconductance = 0.8A/V
is the drop in output voltage during the cur-
= 40.2kΩ) and Switching Waveforms (GSM
is the output voltage, L is the inductor value,
V
DROOP
is the ripple current expected, typically
I
IN
BURST
V
IN(MIN)
OUT
[1 - (V
BURST
CHG
= 3.6V, then R
CH
gm
V
IN
OUT
, but the peak battery current is
= 18kΩ) in Typical Operating
1 - D
CHG
= 2.66A, V
/ V
OUT
V
CHG
IN(MIN)
D
GSM
GSM
V
REF
)] / (L · I
Inductor Selection
resistor
CHG
CHG
DROOP
gm
IN
+ 0.1
RIPPLE
= 31.5kΩ, using
is the input volt-
FB
is shorter than
= 0.36V, V
1 - tol
)
IN
9