MAX1956ETI Maxim Integrated Products, MAX1956ETI Datasheet - Page 15

• Manufacturer Part Number: MAX1956ETI
• Description: DC/DC Switching Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1956ETI

• Number Of Outputs: 2
• Output Voltage: 0.8 V to 4.95 V
• Input Voltage: 1.6 V to 5.5 V
• Package / Case: TQFN EP-28
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The output voltage ripple due to the ESL of the output
capacitor is:
I
These equations are suitable for initial capacitor selec-
tion to meet the ripple requirement, but final values can
depend on the relationship between the LC double-pole
frequency and the capacitor ESR zero. Generally, the
ESR zero is higher than the LC double pole. However, it
is preferable to keep the ESR zero as close to the LC
double pole as possible to negate the sharp phase shift
of the typically high-Q double-LC pole (see the
Compensation Design section). Solid polymer electrolytic
capacitors are recommended because of their low ESR
and ESL at the switching frequency. Higher output-cur-
rent applications require multiple output capacitors con-
nected in parallel to meet the output ripple voltage
requirements.
The response to a load transient depends on the output
capacitor. After a load transient, the output voltage
instantly changes by ESR x ∆I
the controller can respond, the output deviates further,
depending on the inductor and output capacitor values.
After a short time (see the Typical Operating
Characteristics), the controller responds by regulating
the output voltage back to its nominal state. The
response time depends on the closed-loop bandwidth.
With a higher bandwidth, the response is faster, thus
preventing the output voltage from deviating further from
its nominal value. Do not exceed the capacitor’s voltage
or ripple-current ratings.
The MAX1955/MAX1956 drive external, logic-level, N-
channel MOSFETs as the circuit-switch elements. The
key selection parameters:
On-resistance (R
Maximum drain-to-source voltage (V
at least 20% higher than input supply rail at the high-
side MOSFET’s drain.
Gate charges (Q
Choose the MOSFETs with rated R
4.5V. For a good compromise between efficiency and
cost, choose the high-side MOSFET that has a conduction
P-P
is the peak-to-peak inductor current:
V
RIPPLE ESL
I
P P
-
G
DS(ON)
______________________________________________________________________________________
, Q
=
(
180° Out-of-Phase Step-Down Controllers
V
GD
IN
f
SW
)
1.6V to 5.5V Input, 0.5% Accurate, Dual
=
, Q
-
): the lower the better.
V
×
OUT
V
GS
IN
L
LOAD
): the lower the better.



ESL
MOSFET Selection
×
ESL
V
+ ESL x dI/dt. Before
OUT
V
IN
+
DS(ON)
DSS
L



): should be
at V
GS
=
loss equal to switching loss at nominal input voltage
and maximum output current (see below). For low-side
MOSFET, make sure that it does not spuriously turn on
because of dV/dt caused by high-side MOSFET turning
on, as this would result in shoot-through current
degrading the efficiency. MOSFETs with a lower Q
to-Q
For proper thermal-management design, calculate the
power dissipation at the desired maximum operating
junction temperature, maximum output current, and
worst-case input voltage (for low-side MOSFET, worst
case is at VIN(MAX); for high-side MOSFET, it could be
either at VIN(MIN) or VIN(MAX)). High-side MOSFET
and low-side MOSFET have different loss components
due to the circuit operation. Low-side MOSFET oper-
ates as a zero voltage switch; therefore, major losses
are: the channel conduction loss (P
diode conduction loss (P
(P
Use R
where V
the dead time (~25ns), and f
quency.
Because of the zero-voltage switch operation, low-side
MOSFET gate-drive loss occurs as a result of charging
and discharging the input capacitance, (C
loss is distributed among the average DL gate driver’s
pullup and pulldown resistance, (R
the internal gate resistance (R
(~2Ω). The drive power dissipated is given by:
High-side MOSFET operates as a duty-cycle control
switch and has the following major losses: the channel
conduction loss (P
loss (P
MOSFET does not have body-diode conduction loss
because the diode never conducts current:
LSDR
P
GS
LSDR
DS(ON)
P
):
HSSW
ratio have higher immunity to dV/dt.
LSCC
F
P
is the body-diode forward-voltage drop, t
=
HSCC
P
LSDC
), and the drive loss (P
C
at T
=
ISS



1
J(MAX)
=
=
-
×
HSCC
V
(
2
V
V
OUT
V
I
V
OUT
IN
GS
LOAD
IN
:
), the VI overlapping switching
)
LSDC

 ×

2
×
×
×
(
f
V
(
SW
I
), and the gate-drive loss
I
LOAD
F
SW
LOAD
×
GATE
×
t
is the switching fre-
DT
DL
)
)
2
R
2
) of the MOSFET
HSDR
LSCC
×
(0.68Ω typ)), and
GATE
×
×
f
SW
R
R
R
GATE
DS ON
DS ON
), the body-
). High-side
+
( )
( )
ISS
R
DL
). This
DT
GD
15
is
-