LM34919BTLX National Semiconductor Corporation, LM34919BTLX Datasheet - Page 14

• Manufacturer Part Number: LM34919BTLX
• Description: Lm34919b Ultra Small 40v, 600 Ma Constant On-time Buck Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
N - Channel Buck Switch and Driver
The LM34919B integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The peak current
allowed through the buck switch is 1.5A, and the maximum
allowed average current is 1A. 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 forced by the LM34919B
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 10.5 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
der-voltage lockout threshold, or if the RON/SD pin is ground-
ed.
Thermal Shutdown
The LM34919B 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. This feature helps
prevent catastrophic failures from accidental device over-
heating. When the junction temperature reduces below 155°
C (typical hysteresis = 20°C) normal operation resumes.
FIGURE 4. Inductor Current - Current Limit Operation
CC
CC
through the inter-
is below the un-
CC
reaches
14
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. Referring to the
Block Diagram, the circuit is to be configured for the following
specifications:
- V
- V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 1.5 MHz
- Soft-start time = 5 ms
R1 and R2: These resistors set the output voltage. The ratio
of the feedback resistors is calculated from:
For this example, R1/R2 = 0.32. R1 and R2 should be chosen
from standard value resistors in the range of 1.0 kΩ - 10 kΩ
which satisfy the above ratio. For this example, 2.49kΩ is
chosen for R2 and 787Ω for R1.
R
switching frequency. The switching frequncy must be less
than 1.53 MHz to ensure the minimum forced on-time does
not interfere with the circuit's proper operation at the maxi-
mum input voltage. The R
following equation, using the minimum input voltage.
Check that this value resistor does not set an on-time less
than 90 ns at maximum V
A standard value 28 kΩ resistor is used, resulting in a nominal
frequency of 1.49 MHz. The minimum on-time is
Vin = 24V, and the maximum on-time is
Alternately, R
cific on-time is required.
ON
OUT
IN
: This resistor sets the on-time, and (by default) the
= 6V to 24V
= 3.3V
ON
can be determined using Equation 4 if a spe-
R1/R2 = (V
IN
ON
.
OUT
resistor is calculated from the
/2.5V) - 1
≊
424 ns at Vin = 6V.
30102714
≊
129 ns at