LM34910EVAL National Semiconductor, LM34910EVAL Datasheet - Page 8

• Manufacturer Part Number: LM34910EVAL
• Description: BOARD EVALUATION LM34910
• Manufacturer: National Semiconductor

Specifications of LM34910EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 8 ~ 36V
• Regulator Topology: Buck
• Frequency - Switching: 850kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34910
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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ON-Time Timer, and Shutdown
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current through the free-wheeling diode
The current limit threshold can be increased by connecting
an external resistor between S
resistor will typically be less than 1Ω. The peak current out of
SW and I
of SW must be less than 3A, and the average current out of
I
exceed 3.5A, and I
N - Channel Buck Switch and
Driver
The LM34910 integrates an N-Channel buck switch and
associated floating high voltage gate driver. The peak cur-
rent allowed through the buck switch is 3.5A, and the maxi-
mum allowed average current is 3A. The gate driver circuit
works in conjunction with an external bootstrap capacitor
and an internal high voltage diode. A 0.022 µF capacitor (C4)
connected between BST and SW provides the voltage to the
driver during the on-time. During each off-time, the SW pin is
SEN
(Continued)
must be less than 2A. Therefore I
SEN
FIGURE 6. Shutdown Implementation
must not exceed 3.5A. The average current out
OCL
must not exceed 2A.
GND
FIGURE 7. Inductor Current - Current Limit Operation
and I
PK
in Figure 7 must not
SEN
. The external
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8
(D1). Referring to Figure 1, when the buck switch is turned
off the inductor current flows through the load, into S
through the sense resistor, out of I
current 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
is below 2.5V. 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
normal operation the load current is Io, the average of the
ripple 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 (assuming the voltage at FB is
on-time the current ramps up an amount equal to:
During this time the LM34910 is in a constant current mode,
with an average load current (I
at approximately -1V, and C4 charges from V
internal diode. The minimum off-time of 280 ns ensures a
minimum time each cycle to recharge the bootstrap capaci-
tor.
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
reaches 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.
An internal switch grounds the SS pin if V
under-voltage lockout threshold, if a thermal shutdown oc-
curs, or if the R
ON
∆I = (V
/SD pin is grounded.
SEN
is below 1.25A and the voltage at FB
IN
- V
OUT
OCL
) x t
SEN
) equal to 1.25A + ∆I/2.
ON
and through D1. If that
<
/ L1
2.5V). During each
CC
CC
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is below the
through the
GND
CC
,