MAX1639ESE Maxim Integrated Products, MAX1639ESE Datasheet - Page 12

• Manufacturer Part Number: MAX1639ESE
• Description: DC/DC Switching Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1639ESE

• Number Of Outputs: 1
• Output Voltage: 1.1 V to 4.5 V
• Input Voltage: 4.5 V to 5.5 V
• Mounting Style: SMD/SMT
• Package / Case: SOIC-16
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
Resistor RC1 sets a zero that can be used to compen-
sate for the sampling pole generated by the switching
frequency. Set RC1 to the following:
The CC1 pin’s output resistance is 10kΩ.
Compensate the slow-voltage feedback loop by adding
a ceramic capacitor from the CC2 pin to AGND. This is
an integrator loop used to cancel out the DC load-
regulation error. Selection of capacitor CC2 sets the
dominant pole and a compensation zero. The zero is typ-
ically used to cancel the unwanted pole generated by the
load and output filter capacitor at the maximum load cur-
rent. Select CC2 to place the zero close to or slightly
lower than the frequency of the unwanted pole, as fol-
lows:
The transconductance of the integrator amplifier at CC2
is 1mmho. The voltage swing at CC2 is internally
clamped around 2.4V to 3V minimum and 4V to V
maximum to improve transient response times. CC2
can source and sink up to 100µA.
The two high-current N-channel MOSFETs must be
logic-level types with guaranteed on-resistance specifi-
cations at V
better (i.e., 2V max rather than 3V max). Gate charge
should be less than 200nC to minimize switching losses
and reduce power dissipation.
I
power dissipation and are distributed between the
high- and low-side MOSFETs according to duty factor,
as follows:
12
2
P
R losses are the greatest heat contributor to MOSFET
D
(
______________________________________________________________________________________
high side
and Canceling the Load and Output Filter Pole
CC
2
GS
=
Choosing the MOSFET Switches
)
CC
= 4.5V. Lower gate-threshold specs are
RC
1
=
mmho x C
1
1
I
LOAD
=
=
C
4



2
2
OUT
1
f
OSC
+
Setting the Dominant Pole
x R
OUT
10
V
x R
k
DS ON
OUT
V
Ω
IN
x CC
(
ESR
x



I
)
1
OUT MAX
x
V
OUT
(
V
V
OUT
IN
)
CC
Gate-charge losses are dissipated in the IC, and do not
heat the MOSFETs. Ensure that both MOSFETs are at a
safe junction temperature by calculating the temperature
rise according to package thermal-resistance specifica-
tions. The high-side MOSFET’s worst-case dissipation
occurs at the maximum output voltage and minimum
input voltage. For the low-side MOSFET, the worst case
is at the maximum input voltage when the output is short-
circuited (consider the duty factor to be 100%).
Power dissipation in the IC is dominated by average
gate-charge current into both MOSFETs. Average cur-
rent is approximately:
where I
charge for each MOSFET, and f
frequency.
Power dissipation of the IC is:
where I
Junction temperature for the IC is primarily a function of
the PC board layout, since most of the heat is removed
through the traces connected to the pins and the
ground and power planes. A 16-pin narrow SO on a
typical four-layer board with ground and power planes
show equivalent junction-to-ambient thermal
impedance of (θ
ture of the die is approximately:
where T
The rectifier diode D1 is a clamp that catches the nega-
tive inductor swing during the 30ns typical dead time
between turning off the high-side MOSFET and turning
on the low-side MOSFET synchronous rectifier. D1 must
be a Schottky diode, to prevent the MOSFET body
diode from conducting. It is acceptable to omit D1 and
let the body diode clamp the negative inductor swing,
but efficiency will drop about 1%. Use a 1N5819 diode
for loads up to 3A, or a 1N5822 for loads up to 10A.
A signal diode, such as a 1N4148, works well for D2 in
most applications, although a low-leakage Schottky
diode provides slightly improved efficiency. Do not use
P
D
(
low side
CC
A
DD
is the ambient temperature.
is the quiescent supply current of the IC.
is the drive current, Q
Calculating IC Power Dissipation
)
P
I
D
DD
=
= I
JA
Adding the BST Supply Diode
Selecting the Rectifier Diode
T
I
= (Q
LOAD
) about 80°C/W. Junction tempera-
J
CC
= P
G1
x V
2
D
CC
+ Q
x θ
x R
JA
+ I
G2
DS ON
DD
+ T
) x f
(
OSC
A
G
x V
OSC
)
and Capacitor
is the total gate
is the switching
DD
x



1
−
V
OUT
V
IN


