LM3100MH/NOPB National Semiconductor, LM3100MH/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM3100MH/NOPB
• Description: IC REG SIMPLE SW 1.5A 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM3100MH/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 1.5A
• Frequency - Switching: 1MHz
• Voltage - Input: 4.5 ~ 36 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 Down
• Pin Count: 20
• Input Voltage: 36V
• Output Voltage: 0.8 to 7V
• Switching Freq: 1000KHz
• Output Current: 1.5A
• Efficiency: 86%
• Package Type: TSSOP EP
• Output Type: Adjustable
• Switching Regulator: Yes
• Mounting: Surface Mount
• Input Voltage (min): 4.5V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• For Use With: LM3100EVAL - BOARD EVAL LM3100
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant
• Other names: *LM3100MH; *LM3100MH/NOPB; LM3100MH

www.national.com
C
filtering and stability, but also prevents false triggering of the
V
son, C
should be a good quality, low ESR, ceramic capacitor.
C
Experimentation is usually necessary to determine the mini-
mum value for C
larger value. A load which creates significant transients re-
quires a larger value for C
C
high frequency noise at V
cated close to the LM3100.
C
current during the on-time, and limit the voltage ripple at V
assume the voltage source feeding V
impedance greater than zero. If the source’s dynamic
impedance is high (effectively a current source), C
the average input current, but not the ripple current.
At maximum load current, when the buck switch turns on, the
current into V
inductor’s ripple current, ramps up to the peak value, then
drop to zero at turn-off. The average current during the on-
time is the load current. For a worst case calculation, C
supply this average load current during the maximum on-time.
C
where I
and ΔV is the allowable ripple voltage at V
C
long lead inductance at V
capacitor is recommended, located close to the LM3100.
C
ity ceramic capacitor with low ESR is recommended as
VCC
CC
O
O3
IN
IN
IN3
BST
and C
is calculated from:
and C
’s purpose is to help avoid transients and ringing due to
UVLO at the buck switch on/off transitions. For this rea-
is a small value ceramic capacitor to further suppress
: The recommended value for C
: The capacitor on the V
VCC
OUT
O3
should be no smaller than 680 nF for stability, and
IN3
is the load current, t
: C
Inductor Selector for V
: C
IN
O
suddenly increases to the lower peak of the
IN
should generally be no smaller than 10 µF.
O
’s purpose is to supply most of the switch
, as the nature of the load may require a
IN
O
OUT
. A low ESR, 0.1µF ceramic chip
than a fixed load.
CC
. A 47nF is recommended, lo-
output provides not only noise
ON
is the maximum on-time,
BST
OUT
is 33 nF. A high qual-
= 0.8V
IN
IN
.
has an output
20174737
IN
supplies
IN
must
(8)
IN
,
14
C
at turn-on. A low ESR also helps ensure a complete recharge
during each off-time.
C
time, i.e. the time for the reference voltage at the regulation
comparator, and the output voltage, to reach their final value.
The time is determined from the following:
C
pacitor is needed for Discontinuous Conduction Mode to im-
prove the output ripple performance, the recommended value
for C
PC BOARD LAYOUT
The LM3100 regulation, over-voltage, and current limit com-
parators are very fast, and will respond to short duration noise
pulses. Layout considerations are therefore critical for opti-
mum performance. The layout must be as neat and compact
as possible, and all external components must be as close as
possible to their associated pins. Refer to the functional block
diagram, the loop formed by C
es internal to the IC, and the PGND pin should be as small as
possible. The PGND connection to Cin should be as short and
direct as possible. There should be several vias connecting
the Cin ground terminal to the ground plane placed as close
to the capacitor as possible. The boost capacitor should be
connected as close to the SW and BST pins as possible. The
feedback divider resistors and the C
located close to the FB pin. A long trace run from the top of
the divider to the output is generally acceptable since this is
a low impedance node. Ground the bottom of the divider di-
rectly to the GND (pin 7). The output capacitor, C
be connected close to the load and tied directly into the
ground plane. The inductor should connect close to the SW
pin with as short a trace as possible to help reduce the po-
tential for EMI (electro-magnetic interference) generation.
If it is expected that the internal dissipation of the LM3100 will
produce excessive junction temperatures during normal op-
eration, good use of the PC board’s ground plane can help
considerably to dissipate heat. The exposed pad on the bot-
tom of the IC package can be soldered to a ground plane and
that plane should extend out from beneath the IC to help dis-
sipate the heat. The exposed pad is internally connected to
the IC substrate. Additionally the use of thick copper traces,
where possible, can help conduct heat away from the IC. Us-
ing numerous vias to connect the die attach pad to an internal
ground plane is a good practice. Judicious positioning of the
PC board within the end product, along with the use of any
available air flow (forced or natural convection) can help re-
duce the junction temperature.
BST
SS
FB
: The capacitor at the SS pin determines the soft-start
: If output voltage is higher than 1.6V, this feedback ca-
FB
supplies a surge current to charge the buck switch gate
is 10 nF.
IN
, the high and low-side switch-
FB
capacitor should be
OUT
, should