LM34910CSD National Semiconductor, LM34910CSD Datasheet - Page 8

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

www.national.com
the sense resistor, out of I
exceeds 1.25A the current limit comparator output switches
to delay the start of the next on-time period if the voltage at
FB is below 2.5V. The next on-time starts when the current
out of I
If the overload condition persists causing the inductor current
to exceed 1.25A during each on-time, that is detected at the
beginning of each off-time. The operating frequency may be
lower due to longer-than-normal off-times.
Figure 7 illustrates the inductor current waveform. During nor-
mal operation the load current is Io, the average of the ripple
The current limit threshold can be increased by connecting an
external resistor between S
will typically be less than 1Ω. The peak current out of SW and
I
must be less than 3A, and the average current out of I
be less than 2A. Therefore I
3.5A, and I
N - Channel Buck Switch and Driver
The LM34910C integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The peak current
allowed through the buck switch is 3.5A, and the maximum
allowed average current is 3A. The gate driver circuit works
in conjunction with an external bootstrap capacitor and an in-
ternal high voltage diode. A 0.022 µF capacitor (C4) connect-
ed between BST and SW provides the voltage to the driver
during the on-time. During each off-time, the SW pin is at ap-
proximately -1V, and C4 charges from V
nal diode. The minimum off-time of 280 ns ensures a
minimum time each cycle to recharge the bootstrap capacitor.
Softstart
The softstart feature allows the converter to gradually reach
a steady state operating point, thereby reducing start-up
stresses and current surges. Upon turn-on, after V
the under-voltage threshold, an internal 11.5 µA current
source charges up the external capacitor at the SS pin to
2.5V. The ramping voltage at SS (and the non-inverting input
of the regulation comparator) ramps up the output voltage in
a controlled manner.
SEN
must not exceed 3.5A. The average current out of SW
SEN
is below 1.25A and the voltage at FB is below 2.5V.
OCL
must not exceed 2A.
SEN
GND
PK
and through D1. If that current
and I
in Figure 7 must not exceed
FIGURE 7. Inductor Current - Current Limit Operation
SEN
. The external resistor
CC
through the inter-
CC
SEN
reaches
must
8
waveform. When the load resistance decreases the current
ratchets up until the lower peak reaches 1.25A. During the
Current Limited portion of Figure 7, the current ramps down
to 1.25A during each off-time, initiating the next on-time (as-
suming the voltage at FB is <2.5V). During each on-time the
current ramps up an amount equal to:
During this time the LM34910C is in a constant current mode,
with an average load current (I
An internal switch grounds the SS pin if V
der-voltage lockout threshold, if a thermal shutdown occurs,
or if the R
Thermal Shutdown
The LM34910C should be operated so the junction tempera-
ture does not exceed 125°C. If the junction temperature in-
creases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175°C, takes the controller to a low power
reset state by disabling the buck switch and the on-timer, and
grounding the Softstart pin. This feature helps prevent catas-
trophic failures from accidental device overheating. When the
junction temperature reduces below 155°C (typical hysteresis
= 20°C), the Softstart pin is released and normal operation
resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components.
R1 and R2: The ratio of these resistors is calculated from:
R1 and R2 should be chosen from standard value resistors in
the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio.
R
ON
: The minimum value for R
ON
/SD pin is grounded.
ΔI = (V
R1/R2 = (V
IN
- V
OUT
OUT
OCL
ON
) x t
/2.5V) - 1
is calculated from:
) equal to 1.25A + ΔI/2.
ON
/ L1
CC
is below the un-
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