MAX1687ESA 데이터 시트보기 (PDF) - Maxim Integrated

부품명

상세내역

일치하는 목록

MAX1687ESA

Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM

Maxim Integrated 

Step-Up DC-DC Converters with

Precise, Adaptive Current Limit for GSM

a)

b)

MAX1687

REF LIM

MAX1687

DAC

LIM

c)

REF

R3

MAX1687

LIM

R4

VLIM(CHG)

=

VREF

R4

R4 + R3

R3 + R4 > 125kΩ

Figure 5. Current-Limit Adjust

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:

( ( ) ) RCHG

=





IBURST ⋅

VIN(MIN)

VOUT

⋅ 1-

⋅ DGSM

D GSM









+ 0.1



⋅





VIN(MIN)





( ) 



VDROOP

⋅ gmCHG

⋅ VREF

⋅ gmFB

⋅

1

-

tol





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 40.2kΩ RCHG is shorter than

that with an 18kΩ RCHG, 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).

_______________________________________________________________________________________ 9

Share Link: