LM34919EVAL National Semiconductor, LM34919EVAL Datasheet - Page 11

• Manufacturer Part Number: LM34919EVAL
• Description: BOARD EVAL FOR LM34919 SW REG
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM34919EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 600mA
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 800kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34919
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

N - Channel Buck Switch and Driver
The LM34919 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 LM34919
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 LM34919 should be operated so the junction temperature
does not exceed 125°C. If the junction temperature increases,
an internal Thermal Shutdown circuit, which activates (typi-
cally) at 175°C, takes the controller to a low power reset state
by disabling the buck switch. This feature helps prevent catas-
trophic failures from accidental device overheating. 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
11
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:
- VOUT = 5V
- VIN = 8V to 40V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 800 kHz
- 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 = 1. 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 R1 and R2.
R
switching frequency. The switching frequncy must be less
than 1.6 MHz to ensure the minimum forced off-time does not
interfere with the circuit's proper operation. The R
is calculated from the following equation, using the minimum
input voltage.
Check that this value resistor does not set an on-time less
than 120 ns at maximum V
A standard value 43.2 kΩ resistor is used, resulting in a nom-
inal frequency of 806 kHz. The minimum on-time is
at Vin = 40V, and the maximum on-time is
8V. Alternately, R
specific on-time is required.
ON
: This resistor sets the on-time, and (by default) the
ON
R1/R2 = (V
can be determined using Equation 4 if a
IN
.
OUT
/2.5V) - 1
≊
875 ns at Vin =
30004414
www.national.com
ON
≊
resistor
231 ns