Rainbow Electronics MAX849 - Manual (Page 14)

MAX848/MAX849
1-Cell to 3-Cell, High-Power,
Low-Noise, Step-Up DC-DC Converters
14 ______________________________________________________________________________________
Power-On Reset Delay
Adding a timing capacitor from POK to GND generates
a power-on reset delay. The reset time constant is
determined by the pull-up resistor and timing capacitor
(Figure 11). When power is turned on, POK is low and
the capacitor is shorted. When the output voltage
reaches regulation, POK goes high and the capacitor
slowly charges to the output voltage.
The timing resistor value depends on the controller’s
RESET input leakage current. The voltage drop across
the timing resistor should not exceed the difference
between the output voltage and the µC reset threshold
voltage. This resistor should be large enough to mini-
mize the shutdown current.
µC-Controlled Shutdown
The MAX848/MAX849 turn on when ON1 = 1 or ON2 = 0.
The µC monitors the battery voltage and turns off the
device (forces ON1 low and ON2 high) when the bat-
tery is weak.
Layout Considerations
Due to high inductor current levels and fast switching
waveforms, which radiate noise, proper PC board lay-
out is essential. Protect sensitive analog grounds by
using a star ground configuration. Minimize ground
noise by connecting PGND, the input bypass capacitor
ground lead, and the output filter capacitor ground lead
to a single point (star ground configuration). Also, mini-
mize lead lengths to reduce stray capacitance and
trace resistance.
If an external resistor divider is used to set the output
voltage (Figure 5), the trace from FB to the resistors
must be extremely short and must be shielded from
switching signals, such as CLK, DATA, or LX.
MAX848
MAX849
OUT
POK
V
CC
RESET
µ C
R
C
Figure 11. Power-On Reset Delay
Table 4. Component Selection Guide
PRODUCTION INDUCTORS CAPACITORS DIODES
Surface Mount
Sumida CDR63B, CD73, CDR73B, CD74B series
Coilcraft DO1608, DO3308, DT3316 series
Matsuo 267 series
Sprague 595D series
AVX TPS series
Motorola MBR0520L
Through Hole Sumida RCH654 series
Sanyo OS-CON series
Nichicon PL series
Motorola 1N5817
Table 5. Component Suppliers
SUPPLIER PHONE FAX
AVX
USA: 803-946-0690
800-282-4975
803-626-3123
Coilcraft USA: 847-639-6400 847-639-1469
Matsuo USA: 714-969-2491 714-960-6492
Motorola USA: 602-303-5454 602-994-6430
Sanyo
USA: 619-661-6835
Japan: 81-7-2070-6306 619-661-1055
81-7-2070-1174
Sumida
USA: 847-956-0666
Japan: 81-3-3607-5111 847-956-0702
81-3-3607-5144
Chip Information
TRANSISTOR COUNT: 2059