MAX1623 Maxim, MAX1623 Datasheet - Page 8

• Manufacturer Part Number: MAX1623
• Description: 3A / Low-Voltage / Step-Down Regulator with Synchronous Rectification and Internal Switches
• Manufacturer: Maxim

The internal integrator amplifier effectively eliminates any
long-term error within the time constant set by the G
the transconductance amplifier and the capacitor con-
nected to COMP. However, there remains a short-term
load-regulation error in response to load current
changes. Proper FBSEL connection selects the relative
level of current feedback to voltage feedback, which
results in an AC load-regulation error of either 1% or 2%
of the output voltage (Table 1). The 2% setting is auto-
matically selected in preset output voltage mode (FBSEL
connected to V
minimizes the size and cost of the output filter capacitor
required. For extremely tight specifications that cannot
tolerate 2% short-term errors, connect FBSEL to ground
(adjustable mode) for 1% AC load regulation. (See Input
and Output Filter Capacitors (C1, C2) section.)
Synchronous rectification improves efficiency by 3% to
5% at heavy loads when compared to a conventional
Schottky rectifier. To prevent cross-conduction or “shoot-
through,” the synchronous rectifier turns on following a
short delay (dead time) after the P-channel power MOS-
FET turns off. In discontinuous (light-load) mode, the syn-
chronous rectifier switch turns off as the inductor current
approaches zero. The synchronous rectifier works under
all operating conditions, including Idle Mode.
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
Figure 2. Standard 3.3V/3A Application Circuit
8
_______________________________________________________________________________________
4.5V TO 5.5V
NOTE: HEAVY LINES
DENOTE HIGH SWITCHING
CURRENT PATHS.
CC
INPUT
or unconnected). This gain setting
Synchronous Rectification
Setting the AC Loop Gain
10
4.7 F
SHDN
FBSEL
V
FB
CC
MAX1623
m
of
COMP
PGND
TOFF
GND
REF
LX
IN
An internal transconductance amplifier fine tunes the
output DC accuracy. The transconductance amplifier is
compensated at COMP. A capacitor from COMP to
ground determines the gain-bandwidth product and the
overall loop response. This integrator effectively elimi-
nates any long-term error within the time constant set
by the G
capacitor connected to COMP.
For stability, choose COMP as follows:
where G
Table 1. Output Voltage Selection
Unconnected
FBSEL PIN
10 F
0.1 F
GND
V
IN
REF
m
4.7 H
110k
m
C
= 9.1µ
COMP
of the transconductance amplifier and the
220 F
C1
470pF
Ω
REGULATION (%)
.
Integrator Amplifier (COMP)
G
AC LOAD
m
C2
330 F
2
2
1
2
R
LOAD
4
3.3V OUTPUT
C
OUT
VOLTAGE (V)
Adjustable
Adjustable
OUTPUT
2.525
3.33