LM25574EVAL National Semiconductor, LM25574EVAL Datasheet - Page 11

• Manufacturer Part Number: LM25574EVAL
• Description: BOARD EVALUATION FOR LM25574
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r

Specifications of LM25574EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 500mA
• Voltage - Input: 7 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25574
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM25574EVAL

The sample & hold DC level illustrated in Figure 6 is derived
from a measurement of the re-circulating Schottky diode an-
ode current. The re-circulating diode anode should be con-
nected to the IS pin. The diode current flows through an
internal current sense resistor between the IS and PGND
pins. The voltage level across the sense resistor is sampled
and held just prior to the onset of the next conduction interval
of the buck switch. The diode current sensing and sample &
hold provide the DC level of the reconstructed current signal.
The positive slope inductor current ramp is emulated by an
external capacitor connected from the RAMP pin to AGND
and an internal voltage controlled current source. The ramp
current source that emulates the inductor current is a function
of the Vin and Vout voltages per the following equation:
Proper selection of the RAMP capacitor depends upon the
selected value of the output inductor. The value of C
be selected from: C
of the output inductor in Henrys. With this value, the scale
factor of the emulated current ramp will be approximately
equal to the scale factor of the DC level sample and hold
(2.0V / A). The C
to the device and connected directly to the pins of the IC
(RAMP and AGND).
For duty cycles greater than 50%, peak current mode control
circuits are subject to sub-harmonic oscillation. Sub-harmonic
I
RAMP
RAMP
= (10µ x (Vin – Vout)) + 50µA
RAMP
capacitor should be located very close
= L x 5 x 10
-6
FIGURE 6. Composition of Current Sense Signal
, where L is the value
RAMP
can
11
oscillation is normally characterized by observing alternating
wide and narrow pulses at the switch node. Adding a fixed
slope voltage ramp (slope compensation) to the current sense
signal prevents this oscillation. The 50µA of offset current
provided from the emulated current source adds some fixed
slope to the ramp signal. In some high output voltage, high
duty cycle applications, additional slope may be required. In
these applications, a pull-up resistor may be added between
the V
sation.
For V
Calculate optimal slope current, I
For example, at V
Install a resistor from the RAMP pin to V
R
RAMP
CC
OUT
= V
and RAMP pins to increase the ramp slope compen-
FIGURE 7. R
> 7.5V:
CC
/ (I
OS
OUT
- 50µA)
= 10V, I
RAMP
to V
OS
CC
= 100µA.
OS
20214145
for V
= V
OUT
CC
OUT
:
x 10µA/V.
> 7.5V
20214108
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