LM5116MH/NOPB National Semiconductor, LM5116MH/NOPB Datasheet - Page 14

• 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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The sample-and-hold DC level is derived from a measure-
ment of the recirculating current through either the low-side
MOSFET or current sense resistor. The voltage level across
the MOSFET or sense resistor is sampled and held just prior
to the onset of the next conduction interval of the buck switch.
The current sensing and sample-and-hold provide the DC
level of the reconstructed current signal. The positive slope
inductor current ramp is emulated by an external capacitor
connected from the RAMP pin to the AGND and an internal
voltage controlled current source. The ramp current source
that emulates the inductor current is a function of the VIN and
VOUT voltages per the following equation:
Proper selection of the RAMP capacitor (C
on the value of the output inductor (L) and the current sense
resistor (R
and hold value and the ramp amplitude must have the same
dependence on the load current. That is:
Where g
and A is the current sense amplifier gain (10V/V). The ramp
capacitor should be located very close to the device and con-
nected directly to the pins of the IC (RAMP and AGND).
The difference between the average inductor current and the
DC value of the sampled inductor current can cause instability
for certain operating conditions. This instability is known as
sub-harmonic oscillation, which occurs when the inductor rip-
ple current does not return to its initial value by the start of
next switching cycle. Sub-harmonic oscillation is normally
characterized by observing alternating wide and narrow puls-
es at the switch node. Adding a fixed slope voltage ramp
(slope compensation) to the current sense signal prevents
this oscillation. The 25 µA of offset current provided from the
emulated current source adds the optimal slope compensa-
tion to the ramp signal for a 5V output. For higher output
voltages, additional slope compensation may be required. In
m
S
is the ramp generator transconductance (5µA/V)
). For proper current emulation, the DC sample
I
R
= 5µA/V x (VIN-VOUT) + 25µA
FIGURE 6. Composition of Current Sense Signal
RAMP
) depends up-
14
these applications, a resistor is added between RAMP and
VCC to increase the ramp slope compensation.
The DC current sample is obtained using the CS and CSG
pins connected to either a source sense resistor (R
R
R
helpful to adjust the current sense amplifier gain (A) to a lower
value in order to obtain the desired current limit. Adding ex-
ternal resistors R
sense amplifier gain A becomes:
Current Limit
The LM5116 contains a current limit monitoring scheme to
protect the circuit from possible over-current conditions.
When set correctly, the emulated current sense signal is pro-
portional to the buck switch current with a scale factor deter-
mined by the current limit sense resistor. The emulated ramp
signal is applied to the current limit comparator. If the emu-
lated ramp signal exceeds 1.6V, the current cycle is termi-
nated (cycle-by-cycle current limiting). Since the ramp
amplitude is proportional to V
DS(ON)
DS(ON)
FIGURE 7. R
of the low-side MOSFET. In this case it is sometimes
of the low-side MOSFET. For R
DS(ON)
G
in series with CS and CSG, the current
Current Sensing without Diode
Emulation
30007546
IN
- V
OUT
, if V
DS(ON)
30007547
OUT
is shorted, there
sensing, R
S
) or the
S
=