MAX1673 Datasheet, PDF (8/8 Pages) Maxim Integrated Products – Regulated, 125mA-Output, Charge-Pump DC-DC Inverter

Regulated, 125mA-Output,
Charge-Pump DC-DC Inverter
V_RIPPLE(ESR)
=
8



VIN
– _VOUT_^
f_{OSC }



ESRCOUT
R^2OUT C_FLY



The expression for the ripple component predominantly
due to charge transfer is:
V_RIPPLE(ESR)
=
2



VIN
– _VOUT_^
f_{OSC }



R_OUT
1
(C_FLY
+


C_OUT) _{}
where C_FLY and C_OUT are their respective capacitance
values, ESR_COUT is the equivalent series resistance of
C_OUT, R_OUT is the MAX1673 open-loop output imped-
ance (typically 3.5Ω, and f_OSC is the MAX1673 switch-
ing frequency (typically 350kHz). If ESR_COUT is very
small, as is likely when ceramic capacitors are used,
V_{RIPPLE (TRANSFER)} dominates. If ESR is relatively
large, as with low-cost tantalum capacitors, then V_RIP-
PLE (ESR) ^dominates.
When operating in LIN mode, use the following equa-
tion to approximate peak-to-peak output voltage ripple:
V_RIPPLE =
I_OUT
2 f_OSC C_OUT
+ 2I_OUT ESR_COUT
where C_OUT is the output capacitor value, ESR_COUT is
the output capacitor’s ESR, and f_OSC is the MAX1673
oscillator frequency (typically 350kHz).
To ensure LIN mode stability over the entire tempera-
ture range, choose a low-ESR (no more than 100mΩ)
output capacitance using the following equation:
COUT
=
75
x
10
-
6

R1
R1
+ R2

IOUT
where C_OUT is the output capacitor value, and f_MIN is
the minimum oscillator frequency (250kHz). See Table
1 for a list of suggested capacitor suppliers.
Layout Considerations
The MAX1673’s high oscillator frequency requires good
layout technique, which ensures stability and helps
maintain the output voltage under heavy loads. Take
the following steps to ensure good layout:
• Mount all components as close together as possible.
• Place the feedback resistors R1 and R2 close to the
FB pin, and minimize the PC trace length at the FB
circuit node.
• Keep traces short to minimize parasitic inductance
and capacitance.
• Use a ground plane.
___________________Chip Information
TRANSISTOR COUNT: 386
SUBSTRATE CONNECTED TO: IN
Table 1. Partial Listing of Capacitor Vendors
PRODUCTION METHOD
MANUFACTURER
SERIES
AVX
TPS
Surface-Mount Tantalum
Matsuo
267
Sprague
593D, 595D
AVX
X7R
Surface-Mount Ceramic
Matsuo
X7R
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