Rainbow Electronics MAX1677 - Manual (Page 8)

MAX1677
Compact, High-Efficiency, Dual-Output
Step-Up and LCD Bias DC-DC Converter
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Pin Description
Dual Mode is a trademark of Maxim Integrated Products.
Detailed Description
The MAX1677 is a highly-efficient, dual-output power
supply for battery-powered devices. On-chip are two
complete step-up DC-DC converters to power main
logic and bias an LCD (Figure 1). The main boost con-
verter (MBC) has on-chip P-channel and N-channel
MOSFETs that provide synchronous-rectified voltage
conversion for maximum efficiency at loads up to
300mA. See Table 1 for available output current with
typical battery configurations. The output voltage of the
MBC is factory-preset to 3.3V, or can be set from 2.5V
to 5.5V with external resistors (dual-mode operation).
Either fixed-frequency PWM or low-operating-current
PFM operation can be selected for the MBC using the
CLK/SEL input (Table 2).
The LCD boost converter (LCD) includes an internal N-
channel DMOS switch to generate positive or negative
voltages up to ±28V. The polarity of the LCD output is
set by LCDPOL input (Table 3). Figure 2 shows the
MAX1677 configured for a positive LCD output voltage
with a 3.3V main output. Figure 3 shows the MAX1677
configured for a negative LCD output. LCDPOL also
allows the current limit of LCDLX to be reduced from
350mA to 225mA to allow minimum-size inductors in
low-current LCD applications (typically for LCD loads
<10mA).
Also included in the MAX1677 are a precision 1.25V
reference that sources up to 50µA, logic shutdown con-
trol for the MBC and LCD (the MBC must be on for the
LCD to operate), and a low-battery comparator.
NAME FUNCTION
1 OUT
Output Sense Input. The device is powered from OUT. Bypass to GND with a 0.1µF ceramic capacitor.
Connect OUT to POUT through a 10
Ω series resistor.
2 FB
Dual Mode™ Main Boost Feedback Input. Connect to GND for 3.3V output. Connect a voltage-divider from
OUT to FB to adjust the output in the 2.5V to 5.5V range (Figure 5).
PIN
3 LBI
Low-Battery-Comparator Input. Threshold is 614mV. Set the low-battery trip-point with an external voltage
divider (Figure 7).
4 LBO Open-Drain, Low-Battery Output. LBO is low when LBI is below 614mV, otherwise it remains high.
Sync Clock and PWM Select Input.
CLK/SEL = low: low-power, low-quiescent-current PFM mode.
CLK/SEL = high: low-noise, high-power PWM mode at 300kHz.
CLK/SEL = driven with external clock of 200kHz to 400kHz, synchronized PWM high-power mode.
5 CLK/SEL
10 LCDFB
LCD Feedback Input. Threshold is 1.25V for positive with LCDPOL high, and 0 for negative with
LCDPOL low.
9 GND Ground
8 REF 1.25V Reference Output. Bypass with 0.1µF.
7 LCDPOL LCD Polarity Select Input. Sets LCD boost converter polarity and peak current output (Table 2).
6 LCDON LCD Enable Input. Drive high to turn on LCD boost converter. Main DC-DC must also be on.
11 ON I.C. Enable Input. Drive high to enable the MAX1677.
12 LCDLX LCD Boost 28V Switch Drain
13 LCDGND Source of the Internal N-Channel DMOS LCD Boost-Converter Switch
14 PGND Source of the Internal N-Channel Main Boost-Converter Switch
15 LX Main Output Boost Internal Switch Drain
16 POUT
Boost DC-DC Converter Power Output. Source of internal P-channel MOSFET main boost-converter
synchronous rectifier.