MAX5951ETJ+ Maxim Integrated Products, MAX5951ETJ+ Datasheet - Page 16

• Manufacturer Part Number: MAX5951ETJ+
• Description: IC BUCK PWM CTRLR 12V/5V 32-TQFN
• Manufacturer: Maxim Integrated Products
• Type: Step-Down (Buck)r

Specifications of MAX5951ETJ+

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5.5 V
• Current - Output: 10A
• Frequency - Switching: 100kHz ~ 1MHz
• Voltage - Input: 8 ~ 16 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Power - Output: 2.76W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The allowable deviation of the output voltage during
load transients also affects the choice of capacitance,
its ESR, and its equivalent series inductance (ESL). The
output capacitor supplies the load current during a
load step until the controller responds with a greater
duty cycle. The response time (t
the closed-loop bandwidth of the converter (see the
Compensation Design Guidelines section). The resis-
tive drop across the output capacitor’s ESR, the drop
across the capacitor’s ESL, and the capacitor dis-
charge cause a voltage droop during the load-step.
Use a combination of low-ESR tantalum/aluminum elec-
trolyte and ceramic capacitors for better load transient
and voltage ripple performance. Surface-mount capaci-
tors and capacitors in parallel help reduce the ESL.
Keep maximum output-voltage deviation below the tol-
erable limits of the electronics being powered. Use the
following equations to calculate the required ESR, ESL,
and capacitance value during a load step:
where I
load step, and t
controller.
Connect a 25kΩ to 175kΩ resistor, R
AGND to program the valley current-limit threshold
between 50mV and 350mV. ILIM sources 20µA out to
R
current-limit threshold.
The MAX5951 uses a valley current-sense method for
current limiting. The voltage drop across the low-side
MOSFET due to its on-resistance is used to sense the
inductor current. The voltage drop across the low-side
MOSFET at the valley point and at I
R
which is temperature dependent, I
maximum DC load current, and ∆I
peak inductor current.
12V/5V Input Buck PWM Controller
16
ILIM
DS(ON)
______________________________________________________________________________________
. The resulting voltage divided by 10 is the valley
V
VALLEY
STEP
is the on-resistance of the low-side MOSFET,
is the load step, t
ESR
C
ESL
=
OUT
R
RESPONSE
DS ON
=
=
(
=
∆
∆
I
STEP
V
V
I
STEP
Setting the Current Limit
ESL
ESR
)
( )
I
T
STEP
×
is the response time of the
×
×
t
⎛
⎜
⎝
STEP
STEP
∆
t
I
LOAD MAX
RESPONSE
V
RESPONSE
Q
is the rise time of the
LOAD(MAX)
(
P-P
LOAD(MAX)
ILIM
is the peak-to-
)
, from ILIM to
−
) depends on
∆
I
P P
2
is:
−
⎞
⎟
⎠
is the
The 20µA current source, ILIM reference current, has a
temperature coefficient of 3333ppm/°C. This allows the
valley current-limit threshold:
to track and compensate for the increase in the syn-
chronous MOSFET’s R
ture. MOSFETs typically have a temperature coefficient
range within 3000ppm/°C to 7000ppm/°C. Refer to the
MOSFET data sheet for a device-specific temperature
coefficient.
At a given temperature, the calculated V
be less than the minimum valley current-limit threshold
specified.
Figure 4 illustrates the effect of MAX5951 ILIM refer-
ence current temperature coefficient to compensate for
the variation of the MOSFET R
junction temperature range.
When selecting the MOSFETs, consider the total gate
charge, R
drain-to-source voltage, package thermal impedance,
and desired current limit. The product of the MOSFET
gate charge and on-resistance is a figure of merit, with
a lower number signifying better performance. Choose
MOSFETs optimized for high-frequency switching
applications. The average gate-drive current from the
MAX5951’s output is proportional to the frequency and
gate charge required to drive the MOSFET. The power
dissipated in the MAX5951 is proportional to the input
voltage and the average drive current (see the Power
Dissipation section).
Figure 4. Current-Limit Threshold and R DS(ON) vs.
Temperature
DS(ON)
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
-40
VALLEY CURRENT-LIMIT THRESHOLD
, power dissipation, the maximum
AND R
R
-15
ILIM
Power MOSFET Selection
DS(ON)
DS(ON)
TEMPERATURE (°C)
× 20
10
10
vs. TEMPERATURE
µ ( )
DS(ON)
with increasing tempera-
A T
35
R
over the operating
DS(ON)
60
V
ILIM
VALLEY
85
must