LM5001ISOEVAL National Semiconductor, LM5001ISOEVAL Datasheet - Page 10

• Manufacturer Part Number: LM5001ISOEVAL
• Description: BOARD EVALUATION LM5001ISO
• Manufacturer: National Semiconductor

Specifications of LM5001ISOEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 16 ~ 42V
• Regulator Topology: Flyback
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5001
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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When isolation between primary and secondary circuits is re-
quired, the Error Amplifier is usually disabled by connecting
the FB pin to GND. This allows the COMP pin to be driven
directly by the collector of an opto-coupler. In isolated designs
the external error amplifier is located on the secondary circuit
and drives the opto-coupler LED. The compensation network
is connected to the secondary side error amplifier. An exam-
ple of an isolated regulator with an opto-coupler is shown in
Figure 7.
Current Amplifier and Slope
Compensation
The LM5001 employs peak current mode control which also
provides a cycle-by-cycle over current protection feature. An
internal 50 mΩ current sense resistor measures the current
in the power MOSFET source. The sense resistor voltage is
amplified 30 times to provide a 1.5V/A signal into the current
limit comparator. Current limiting is initiated if the internal cur-
rent limit comparator input exceeds the 1.5V threshold, cor-
responding to 1A. When the current limit comparator is
triggered, the SW output pin immediately switches to a high
impedance state.
The current sense signal is reduced to a scale factor of 1.05V/
A for the PWM comparator signal. The signal is then summed
with a 450mV peak slope compensation ramp. The combined
signal provides the PWM comparator with a control signal that
reaches 1.5V when the MOSFET current is 1A. For duty cy-
cles greater than 50%, current mode control circuits are sub-
ject to sub-harmonic oscillation (alternating between short
and long PWM pulses every other cycle). Adding a fixed slope
voltage ramp signal (slope compensation) to the current
sense signal prevents this oscillation. The 450mV ramp (zero
FIGURE 1. Type II Compensator
10
volts when the power MOSFET turns on, and 450mV at the
end of the PWM clock cycle) adds a fixed slope to the current
sense ramp to prevent oscillation.
To prevent erratic operation at low duty cycle, a leading edge
blanking circuit attenuates the current sense signal when the
power MOSFET is turned on. When the MOSFET is initially
turned on, current spikes from the power MOSFET drain-
source and gate-source capacitances flow through the cur-
rent sense resistor. These transient currents normally cease
within 50 ns with proper selection of rectifier diodes and prop-
er PC board layout.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
IC in the event the maximum junction temperature is exceed-
ed. When the 165°C junction temperature threshold is
reached, the regulator is forced into a low power standby
state, disabling all functions except the VCC regulator. Ther-
mal hysteresis allows the IC to cool down before it is re-
enabled. Note that since the VCC regulator remains function-
al during this period, the soft-start circuit shown in Figure 5
should be augmented if soft-start from Thermal Shutdown
state is required.
Power MOSFET
The LM5001 switching regulator includes an N-Channel
MOSFET with 440 mΩ on-resistance. The on-resistance of
the LM5001 MOSFET varies with temperature as shown in
the Typical Performance Characteristics graph. The typical
total gate charge for the MOSFET is 4.5 nC which is supplied
from the VCC pin when the MOSFET is turned on.
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