LM34910CSD National Semiconductor, LM34910CSD Datasheet - Page 9

• Manufacturer Part Number: LM34910CSD
• Description: High Voltage (50V, 1.25A) Step Down Switching Regulator
• Manufacturer: National Semiconductor

Equation 1 can be used to select R
is desired as long as the above limitation is met.
L1: The main parameter affected by the inductor is the output
current ripple amplitude (I
termined at both the minimum and maximum nominal load
currents.
a) If the maximum load current is less than the current limit
threshold (1.25A), the minimum load current is used to deter-
mine the maximum allowable ripple. To maintain continuous
conduction mode the lower peak should not reach 0 mA. For
this case, the maximum ripple current is:
The ripple calculated in Equation 6 is then used in the follow-
ing equation:
where V
from equation 1. This provides a minimum value for L1. The
next larger standard value should be used, and L1 should be
rated for the I
b) If the maximum load current is greater than the current limit
threshold (1.25A), the LM34910C ensures the lower peak
reaches 1.25A each cycle, requiring that I
the difference. The upper peak, however, must not exceed
3.5A. For this case, the ripple limits are:
and
The lesser of Equations 8 and 9 is then used in Equation 7. If
I
next larger value should then be used for L1. If I
used, the minimum V
value should then be used for L1. L1 must be rated for the
peak value of the current waveform (I
C3: The capacitor on the V
filtering and stability, but also prevents false triggering of the
V
son, C3 should be no smaller than 0.1 µF, and should be a
good quality, low ESR, ceramic capacitor.
C2, and R3: Since the LM34910C requires a minimum of 25
mV
ripple at V
ripple is created by the inductor ripple current acting on C2’s
ESR + R3. The minimum ripple current is calculated using
equation 7, rearranged to solve for I
minimum ESR for C2 is then equal to:
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in Figure 1. Generally R3 is less
than 1Ω. C2 should generally be no smaller than 3.3 µF, al-
though that is dependent on the frequency and the allowable
ripple amplitude at V
sary to determine the minimum value for C2, as the nature of
the load may require a larger value. A load which creates sig-
OR(MAX2)
CC
p-p
UVLO at the buck switch on/off transitions. For this rea-
of ripple at the FB pin for proper operation, the required
IN
is used, the maximum V
is the maximum input voltage and Fs is determined
OUT1
I
I
OR(MIN1)
OR(MAX2)
PK
is increased by R1 and R2. This necessary
current level.
I
OR(MAX1)
IN
OUT1
= 2 x (I
= 2 x (3.5A - I
is used in Equation 7. The next smaller
. Experimentation is usually neces-
OR
= 2 x I
CC
O(max)
). The limits for I
output provides not only noise
O(min)
IN
- 1.25A)
is used in Equation 7. The
ON
O(max)
OR
PK
if a specific frequency
at minimum V
in Figure 7).
)
OR
be at least twice
OR
must be de-
OR(MIN1)
IN
. The
(10)
(6)
(7)
(8)
(9)
is
9
nificant transients requires a larger value for C2 than a non-
varying load.
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck switch
is turned on. The forward voltage drop is significant in the
event the output is short-circuited as it is mainly this diode’s
voltage (plus the voltage across the current limit sense resis-
tor) which forces the inductor current to decrease during the
off-time. For this reason, a higher voltage is better, although
that affects efficiency. A reverse recovery time of
a forward voltage drop of
leakage specification is important as that can significantly af-
fect efficiency. D1’s reverse voltage rating must be at least as
great as the maximum V
or exceed I
C1 and C5: C1’s purpose is to supply most of the switch cur-
rent during the on-time, and limit the voltage ripple at V
the assumption that the voltage source feeding V
output impedance greater than zero. If the source’s dynamic
impedance is high (effectively a current source), it supplies
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, C1 must
supply this average load current during the maximum on-time.
C1 is calculated from:
where Io is the load current, t
ΔV is the allowable ripple voltage at V
help avoid transients and ringing due to long lead inductance
at V
mended, located close to the LM34910C .
C4: The recommended value for C4 is 0.022 µF. A high quality
ceramic capacitor with low ESR is recommended as C4 sup-
plies a surge current to charge the buck switch gate at turn-
on. A low ESR also helps ensure a complete recharge during
each off-time.
C6: The capacitor at the SS pin determines the softstart time,
i.e. the time for the reference voltage at the regulation com-
parator, and the output voltage, to reach their final value. The
time is determined from the following:
PC BOARD LAYOUT
The LM34910C regulation, over-voltage, and current limit
comparators are very fast, and respond to short duration
noise pulses. Layout considerations are therefore critical for
optimum performance. The layout must be as neat and com-
pact as possible, and all of the components must be as close
as possible to their associated pins. The current loop formed
by D1, L1, C2 and the S
as possible. The ground connection from C2 to C1 should be
as short and direct as possible.
If it is expected that the internal dissipation of the LM34910C
will produce excessive junction temperatures during normal
IN
. A low ESR, 0.1 µF ceramic chip capacitor is recom-
PK
IN
Figure 7.
suddenly increases to the lower peak of the
GND
IN
≊
, and its current rating must equal
0.75V are preferred. The reverse
and I
ON
is the maximum on-time, and
SEN
pins should be as small
IN
. C5’s purpose is to
www.national.com
≊
30 ns, and
IN
has an
IN
, on