LM5118MH/NOPB National Semiconductor, LM5118MH/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM5118MH/NOPB
• Description: IC CTLR BUCK-BOOST 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LM5118MH/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 70 V
• Frequency - Switching: 500kHz
• Voltage - Input: 3 ~ 75 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: Step Up/Step Down
• Pin Count: 20
• Input Voltage: 3 to 75V
• Output Voltage: 1.23 to 70V
• Output Current: 3.5A
• Package Type: TSSOP EP
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Package: 20TSSOP EP
• For Use With: LM5118EVAL - BOARD EVALUATION FOR LM5118
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant
• Other names: LM5118MH

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to guarantee a given output ripple voltage. Buck-boost mode
capacitance can be estimated from:
ESR requirements can be estimated from:
For our example, with a ΔVOUT (output ripple) of 50 mV,
C
ESR
If hold-up times are a consideration, the values of input/output
capacitors must be increased appropriately. Note that it is
usually advantageous to use multiple capacitors in parallel to
achieve the ESR value required. Also, it is good practice to
put a .1 µF - .47 µF ceramic capacitor directly on the output
pins of the supply to reduce high frequency noise. Ceramic
capacitors have good ESR characteristics, and are a good
choice for input and output capacitors. It should be noted that
the effective capacitance of ceramic capacitors decreases
with dc bias. For larger bulk values of capacitance, a low ESR
electrolytic is usually used. However, electrolytic capacitors
have poor tolerance, especially over temperature, and the
selected value should be selected larger than the calculated
value to allow for temperature variation. Allowing for compo-
nent tolerances, the following values of Cout were chosen for
this design example:
Two 180 µF Oscon electrolytic capacitors for bulk capaci-
tance
Two 47 µF ceramic capacitors to reduce ESR
Two 0.47 µF ceramic capacitors to reduce spikes at the out-
put .
D1
Reverse recovery currents degrade performance and de-
crease efficiency. For these reasons, a Schottky diode of
appropriate ratings should be used for D1. The voltage rating
of the boost diode should be equal to VOUT plus some mar-
gin. Since D1 only conducts during the buck switch off time in
either mode, the current rating required is:
D4
A Schottky type re-circulating diode is required for all LM5118
applications. The near ideal reverse recovery characteristics
and low forward voltage drop are particularly important diode
characteristics for high input voltage and low output voltage
applications. The reverse recovery characteristic determines
how long the current surge lasts each cycle when the buck
switch is turned on. The reverse recovery characteristics of
Schottky diodes minimize the peak instantaneous power in
the buck switch during the turn-on transition. The reverse
breakdown rating of the diode should be selected for the max-
imum VIN plus some safety margin.
The forward voltage drop has a significant impact on the con-
version efficiency, especially for applications with a low output
voltage. “Rated” current for diodes vary widely from various
manufacturers. For the LM5118 this current is user selectable
through the current sense resistor value. Assuming a worst
MIN
MAX
= 141 µF
= 3.8 mΩ
I
I
DIODE
DIODE
= I
= I
OUT
OUT
x (1-D) Buck Mode
Buck-Boost Mode
18
case 0.6V drop across the diode, the maximum diode power
dissipation can be high. The diode should have a voltage rat-
ing of VIN and a current rating of IOUT. A conservative design
would at least double the advertised diode rating since spec-
ifications between manufacturers vary. For the reference de-
sign a 100V, 10A Schottky in a D2PAK package was selected.
C1 - C5 = INPUT CAPACITORS
A typical regulator supply voltage has a large source
impedance at the switching frequency. Good quality input ca-
pacitors are necessary to limit the ripple voltage at the VIN
pin while supplying most of the switch current during the buck
switch on-time. When the buck switch turns on, the current
into the buck switch steps from zero to the lower peak of the
inductor current waveform, then ramps up to the peak value,
and then drops to the zero at turn-off. The RMS current rating
of the input capacitors depends on which mode of operation
is most critical.
This value is a maximum at 50% duty cycle which corre-
sponds to VIN = 75 volts.
Checking both modes of operation we find:
I
I
Therefore C1 - C5 should be sized to handle 4.7A of ripple
current. Quality ceramic capacitors with a low ESR should be
selected. To allow for capacitor tolerances, four 2.2 µF, 100V
ceramic capacitors will be used. If step input voltage tran-
sients are expected near the maximum rating of the LM5118,
a careful evaluation of the ringing and possible spikes at the
device VIN pin should be completed. An additional damping
network or input voltage clamp may be required in these cas-
es.
C20
The capacitor at the VCC pin provides noise filtering and sta-
bility for the VCC regulator. The recommended value of C20
should be no smaller than 0.1 µF, and should be a good qual-
ity, low ESR, ceramic capacitor. A value of 1 µF was selected
for this design. C20 should be 10 x C8.
If operating without VCCX, then
must be less than the VCC current limit.
C8
The bootstrap capacitor between the HB and HS pins sup-
plies the gate current to charge the buck switch gate at turn-
on. The recommended value of C8 is 0.1 µF to 0.47 µF, and
should be a good quality, low ESR, ceramic capacitor. A value
of 0.1 µF was chosen for this design.
C16 = C
The capacitor at the SS pin determines the soft-start time, i.e.
the time for the reference voltage and the output voltage, to
reach the final regulated value. The time is determined from:
RMS(BUCK)
RMS(BUCK-BOOST)
SS
f
OSC
= 1.5 Amps
x (Q
= 4.7 Amps
C
Buck + Boost) + I
LOAD(INTERNAL)