LM5025AEVAL National Semiconductor, LM5025AEVAL Datasheet - Page 3

• Manufacturer Part Number: LM5025AEVAL
• Description: BOARD EVALUATION LM5025A
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5025AEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 30A
• Voltage - Input: 36 ~ 72V
• Regulator Topology: Buck
• Frequency - Switching: 230kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5025A
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Board Layout and Probing
Figures 1-3 show the board layout, main components, and
critical probe points for testing the LM5115 AC mode evalu-
ation board in conjunction with the LM5025A board. The
following notes should be considered prior to applying power
to the board:
1. Main input power (36V to 72V) is applied to points J1
2. The main current carrying components (LM5115 board:
3. The diameter and length of the wire used to connect the
Board Connections/Start-Up
The input power connections are made to terminals J1 (+)
and J4 (-) of the LM5025A evaluation board. The input
source must be capable of supplying the load on both the
output of the LM5025A board and LM5115 board. The input
to the LM5115 is supplied by the secondary winding of the
LM5025A board. J7 (LM5025A) connects to J1 (LM5115)
and J10 (LM5025A) connects to J2 (LM5115) (see Figure
1-3). The main load is connected to terminals J9 (+) and J5
(-) for the LM5025. Terminals J3 (+) and J4 (-) are the load
connections for the LM5115. Before start-up, a voltmeter
should be connected to the input terminals and to the output
terminals. The input current should be monitored with an
ammeter or a current probe. Soft-start provided by the
LM5115 will insure that the output rises with a smooth turn
on without overshoot (Figure 12). The LM5115 evaluation
board will operate in the continuous conduction mode even
with a light or no load (Figure 4).
Performance LM5115 Secondary
Side Post Regulator
Performance of the LM5115 evaluation board can be seen in
the following figures:
1. Power Conversion Efficiency (Figure 5 & 6)
2. Gate Delays (Figure 7 & 8)
3. Short circuit response (Figure 9)
4. Step Load Response (Figure 10 & 11)
5. Startup and Shutdown Response (Figure 12 & 13)
6. Ripple Voltage (Figure 14)
7. Load Regulation (Figure 15)
8. Secondary Closed Loop Frequency Response (Figure
V
The LM5115 produces a LDO 7V regulated output (VCC)
that can supply up to 40mA of DC current. The VCC regula-
CC
and J4 of the LM5025A board, connected to VIN and
GND respectively.
L1, Q1, and Q2; LM5025A board L2, Q3-Q6) will be hot
to the touch at maximum load current. USE CAUTION.
When operating at load currents in excess of 5A the use
of a fan to provide forced air flow IS NECESSARY.
load is important. To ensure that there is not a significant
voltage drop in the wires, a minimum of 14 gauge wire is
recommended.
16)
3
tor supplies power for the high current gate drive for the low
side MOSFET and the bootstrap capacitor of the high side
MOSFET driver.
Current Limit Operation
Inductor current is sensed through the parallel resistances of
R7, R8, and R9. The resistor values are designed for a
current limit of ~ 9A. Current limiting occurs when the delta
voltage across the sense resistor exceeds 45mV causing the
current sense amplifier to pull down the combined CO and
COMP pins. Pulling COMP low reduces the width of pulses
to the high side driver, limiting the output current of the
converter. After reaching the current limit, the voltage feed-
back causes the COMP pin to rise and turn on the high side
driver until the inductor current again reaches the ~ 9A cur-
rent limit threshold. (Figure 9).
The parallel resistance also serves to inject the inductor
current into the LM5115 feedback loop. Injecting a signal
proportional to the instantaneous inductor current into a
voltage mode controller improves the control loop stability
and bandwidth. Current injection, which is a form of average
current mode control, eliminates the lead R-C network in the
feedback path that is normally required with voltage mode
control. This not only simplifies the compensation but also
reduces sensitivity to output noise that could pass through
the lead network to the error amplifier.
In cases where a noisy current sense is present, adding a
low pass filter to the input of CS and VOUT can help restore
a cleaner waveform (Figure 18 , R16 and C10). Care must
be taken not to have a large RC time constant to avoid
instability.
Foldback Current Limit
Current limit foldback can be implemented with the following
components: R17, R18, C10, D5, and R16 (see Figure 18).
At nominal output voltage (VOUT
biased and the current limit threshold is still ~ 45mV. At lower
output voltage the resistor divider network along with the
forward biased diode (D5) will increase the voltage across
R16. In order to reach the 45mV current limit threshold, the
voltage across the sense resistor (R7-R9) is reduced due to
the increase in voltage across R16. Thus, the current limit is
reduced providing current limit foldback. The resistor divider
sets the voltage when current limit foldback kicks in and R16
sets the amount of current limit foldback.
A most common occurrence, that will prove unnerving, is
when the current limit set on the source supply is insufficient
for the load. The result is similar to having the high source
impedance referred to earlier. The interaction of the source
supply folding back and the UUT going into undervoltage
shutdown will start an oscillation, or chatter, that may have
highly undesirable consequences.
Optional DC Buck
LM5115 can also be configured as a DC buck regulator.
Information for the DC board can be found in LM5115 DC
application note and LM5115 datasheet.
>
3V) D5 is reversed
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