LM5001NISOEVAL National Semiconductor, LM5001NISOEVAL Datasheet - Page 3

• Manufacturer Part Number: LM5001NISOEVAL
• Description: BOARD EVALUATION FOR LM5001
• Manufacturer: National Semiconductor

Specifications of LM5001NISOEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-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: -
• Lead Free Status / Rohs Status: Not Compliant

Flyback Topology
FLYBACK TRANSFORMER
Two things need to be considered when specifying a Flyback
transformer to a magnetics manufacturer (Coilcraft, Pulse
Engineering, Cooper-Coiltronics, etc.), the turns ratio to de-
termine the duty cycle D (MOSFET on time compared to the
switching period) and the primary inductance (L
mine the current sense ramp for current mode control.
To start, the primary inductance in Continuous Current Mode
(CCM) is designed to provide a ramp during the MOSFET on
time, of around 30% of the full load MOSFET current. This
produces a good signal-to-noise ratio for Current Mode Con-
trol.
The CCM duty cycle can be designed for 50% with nominal
input voltage. The transfer function of a Flyback Powerstage
is
So the duty cycle is
And the approximate turns ratio is
The primary inductance (L
POWERSTAGE ANALYSIS
In any switchmode topology that has the power MOSFET be-
tween the inductor and the output capacitor (boost, buck-
boost, Flyback, SEPIC, etc.) a Right Half-Plane Zero (RHPZ)
is produced by the Powerstage in the loop transfer function
during Continuous Conduction Mode (CCM). If the topology
is operated in Discontinuous Conduction Mode (DCM) the
RHPZ does not exist. It is a function of the duty cycle, load
and inductance, and causes an increase in loop gain while
reducing the loop phase margin. A common practice is to de-
termine the worst case RHPZ frequency and set the loop unity
gain frequency below one-third of the RHPZ frequency.
In the Flyback topology, the equation for the RHPZ is:
PRI
) is then
PRI
) to deter-
3
The worst case RHPZ frequency is at the maximum load
where I
the duty cycle D is the highest.
The LM5001 uses Slope Compensation to insure stability
when the duty cycle exceeds 45%. This has the effect of
adding some Voltage Mode control to this Current Mode IC.
The effect on the Powerstage (Plant) transfer function is cal-
culated in the following three equations:
Inductor current slope during MOSFET on time:
Slope Compensation ramp:
Current Mode sampling gain:
The control-to-output transfer function (G
capacitors (ceramic, etc.) is:
If high ESR capacitors (aluminum electrolytic, etc.) are used
for the output capacitance, an additional zero appears at fre-
quency:
which increases the gain slope by +20dB per decade of fre-
quency and boosts the phase 45° at F
x F
With these calculations, an approximate Powerstage Bode
plot can be constructed with:
ZERO(ESR)
OUT
is the highest and at minimum input voltage where
. The output ripple voltage is also increased by:
Se = 450 mV x f
SW
ZERO(ESR)
VC
) using low ESR
and 90° at 10
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