MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
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To set the current limit, apply a voltage of 0 to 1V at
LIM. The current limit is 200mA when VLIM = 0 to
0.25V. Use the following equation to calculate ILIM:
ILIM = VLIM (0.86A/V) – 0.06A
where VLIM = 0.25V to 1V.
VLIM is internally clamped to 1.25V when the voltage
applied at VLIM is above 1.25V. Generate VLIM by one
of three methods: an externally applied voltage, the
output of a DAC, or a resistor-divider using VREF as the
supply voltage (TSSOP packages) (Figure 5). Note that
REF can supply up to 10µA.
Determine VLIM as follows:
VLIM = (ILX(PEAK) + 0.06A) / 0.86
where ILX(PEAK) = [(ILOAD · VOUT) / VIN ] + 0.1A (see
the Inductor Current parameter in the
Typical Operating
Characteristics
).
Setting Recharge Time (MAX1688)
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
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:
RCHG is the external resistor
IBURST is the peak burst current expected
DGSM is the duty cycle of GSM
VIN is the input voltage
VOUT is the output voltage
VREF = 1.25V
VDROOP is the drop in output voltage during the cur-
rent burst
gmCHG is the internal transconductance = 0.8A/V
gmFB is the feedback transconductance = 200µA/V
tol is the tolerance of the RCHG resistor
For example, for IBURST = 2.66A, VDROOP = 0.36V, VIN
= +2.7V, and VOUT = 3.6V, then RCHG = 31.5kΩ, using
a 5% tolerance resistor.
The recovery time for a 40.2kΩRCHG is shorter than
that with an 18kΩRCHG, but the peak battery current is
higher. See Switching Waveforms (GSM Pulsed Load
1A, RCHG = 40.2kΩ) and Switching Waveforms (GSM
Pulsed Load 1A, RCH = 18kΩ) in
Typical Operating
Characteristics
.
Inductor Selection
The value of the inductor determines the switching fre-
quency. Calculate the switching frequency as:
f = VIN [1 - (VIN / VOUT)] / (L · IRIPPLE)
where f is the switching frequency, VIN is the input volt-
age, VOUT is the output voltage, L is the inductor value,
and IRIPPLE is the ripple current expected, typically
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
).