LM5116MH/NOPB National Semiconductor, LM5116MH/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM5116MH/NOPB
• Description: IC CTRLR SYNCH BUCK 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM5116MH/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.22 ~ 80 V
• Current - Output: 20A
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 6 ~ 100 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Dc To Dc Converter Type: Synchronous Buck Controller
• Pin Count: 20
• Input Voltage: 6 to 100V
• Output Voltage: 1.215 to 80V
• Output Current: 3.5A
• Package Type: TSSOP EP
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• For Use With: LM5116-12EVAL - BOARD EVALUATION FOR LM5116-12LM5116EVAL - BOARD EVALUATION LM5116
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant
• Other names: LM5116MH

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During this initial charging of C
voltage, the LM5116 will force diode emulation. That is, the
low-side MOSFET will turn off for the remainder of a cycle if
the sensed inductor current becomes negative. The inductor
current is sensed by monitoring the voltage between SW and
DEMB. As the SS capacitor continues to charge beyond
1.215V to 3V, the DEMB bias current will increase from 0µA
up to 40µA. With the use of an external DEMB resistor
(R
increase resulting in the gradual transition to synchronous
operation. Forcing diode emulation during soft-start allows
the LM5116 to start up into a pre-biased output without un-
necessarily discharging the output capacitor. Full syn-
chronous operation is obtained if the DEMB pin is always
biased to a higher potential than the SW pin when LO is high.
R
which is adequate for most applications. The DEMB bias po-
tential should always be kept below 2V. At very light loads
with larger values of output inductance and MOSFET capac-
itance, the switch voltage may fall slowly. If the SW voltage
does not fall below the DEMB threshold before the end of the
HO fall to LO rise dead-time, switching will default to diode
emulation mode. When R
run in diode emulation.
Once SS charges to 3V the SS latch is set, increasing the
DEMB bias current to 65µA. An amplifier is enabled that reg-
ulates SS to 160mV above the FB voltage. This feature can
prevent overshoot of the output voltage in the event the output
voltage momentarily dips out of regulation. When a fault is
detected (VCC under-voltage, UVLO pin < 1.215, or EN = 0V)
the soft-start capacitor is discharged. Once the fault condition
is no longer present, a new soft-start sequence begins.
HO Ouput
The LM5116 contains a high current, high-side driver and as-
sociated high voltage level shift. This gate driver circuit works
in conjunction with an external diode and bootstrap capacitor.
A 1 µF ceramic capacitor, connected with short traces be-
tween the HB pin and SW pin, is recommended. During the
off-time of the high-side MOSFET, the SW pin voltage is ap-
proximately -0.5V and the bootstrap capacitor charges from
VCC through the external bootstrap diode. When operating
with a high PWM duty cycle, the buck switch will be forced off
each cycle for 450 ns to ensure that the bootstrap capacitor
is recharged.
The LO and HO outputs are controlled with an adaptive dead-
time methodology which insures that both outputs are never
enabled at the same time. When the controller commands HO
to be enabled, the adaptive block first disables LO and waits
for the LO voltage to drop below approximately 25% of VCC.
HO is then enabled after a small delay. Similarly, when HO
turns off, LO waits until the SW voltage has fallen to ½ of VCC.
LO is then enabled after a small delay. In the event that SW
does not fall within approximately 150 ns, LO is asserted high.
This methodology insures adequate dead-time for any size
MOSFET.
Thermal Protection
Internal thermal shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction temper-
ature is exceeded. When activated, typically at 170°C, the
controller is forced into a low power reset state, disabling the
output driver and the bias regulator. This is designed to pre-
vent catastrophic failures from accidental device overheating.
DEMB
DEMB
), the current sense threshold for diode emulation will
= 10kΩ will bias the DEMB pin to 0.45V minimum,
DEMB
= 0Ω, the LM5116 will always
SS
to the internal reference
16
Application Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. The Bill of Mate-
rials for this design is listed in
Figure 16
•
•
•
•
Simplified equations are used as a general guideline for the
design method. Comprehensive equations are provided at
the end of this section.
TIMING RESISTOR
R
frequency applications are smaller but have higher losses.
Operation at 250kHz was selected for this example as a rea-
sonable compromise for both small size and high efficiency.
The value of R
culated as follows:
The nearest standard value of 12.4kΩ was chosen for R
OUTPUT INDUCTOR
The inductor value is determined based on the operating fre-
quency, load current, ripple current and the input and output
voltages.
Knowing the switching frequency (f
rent (I
output voltage (V
The maximum ripple current occurs at the maximum input
voltage. Typically, I
When running diode emulation mode, the maximum ripple
current should be less than twice the minimum load current.
For full synchronous operation, higher ripple current is ac-
T
Output voltage = 5V
Input voltage = 7V to 60V
Maximum load current = 7A
Switching frequency = 250kHz
sets the oscillator switching frequency. Generally, higher
PP
), maximum input voltage (V
is configured for the following specifications:
FIGURE 10. Inductor Current
T
for 250kHz switching frequency can be cal-
OUT
PP
), the inductor value can be calculated:
is 20% to 40% of the full load current.
Table
SW
IN(MAX)
1. The circuit shown in
), maximum ripple cur-
) and the nominal
30007545
T
.