MAX1644 Maxim, MAX1644 Datasheet - Page 7

• Manufacturer Part Number: MAX1644
• Description: Advanced Chemistry-Independent / Level 2 Battery Charger with Input Current Limiting
• Manufacturer: Maxim

Figure 1. Functional Diagram
In a step-down regulator without synchronous rectifica-
tion, an external Schottky diode provides a path for cur-
rent to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS syn-
chronous switch reduces conduction losses and
improves efficiency.
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In
constant-off-time mode, the synchronous-rectifier
switch turns off just prior to the PMOS power switch
turning on. While both switches are off, inductor current
flows through the internal body diode of the NMOS
switch. The internal body diode’s forward voltage is rel-
atively high.
Junction-to-ambient thermal resistance, θ
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1644 evaluation kit
has 0.5 in.
60°C/W with no airflow. Airflow over the IC significantly
reduces the junction-to-ambient thermal resistance. For
Synchronous Rectification and Internal Switches
V
IN
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
2
10Ω
of copper area and a thermal resistance of
470pF
2.2μF
1μF
_______________________________________________________________________________________
COMP
2A, Low-Voltage, Step-Down Regulator with
SHDN
V
REF
Synchronous Rectification
CC
REF
Thermal Resistance
G
m
REF
GND
REF
SELECTION
FEEDBACK
JA
FBSEL
, is highly
COMPARATOR
MAX1644
SUMMING
TIMER
TOFF
R
TOFF
heatsinking purposes, evenly distribute the copper area
connected at the IC among the high-current pins.
Power dissipation in the MAX1644 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to supply current in the control
section and average current used to charge and dis-
charge the gate capacitance of the internal switches
are less than 30mW at 300kHz. This number is reduced
when the switching frequency decreases as the part
enters Idle Mode. Combined conduction losses in the
two power switches are approximated by:
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
where: θ
PWM LOGIC
CURRENT
CURRENT
DRIVERS
SS
SENSE
SENSE
AND
SKIP
T
T
JA
J,MAX
A,MAX
= junction-to-ambient thermal resistance
0.01μF
θ
PGND
JA
= maximum junction temperature
= maximum ambient temperature
= (T
P
D
J,MAX
= I
LX
IN
FB
OUT 2
- T
10μF
A,MAX
· R
Power Dissipation
ON
) / P
V
3.0V TO 5.5V
C
OUT
IN
D
V
OUT
7