LM5005MH National Semiconductor, LM5005MH Datasheet - Page 13

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

RAMP Generator
The Sample and Hold DC level illustrated in Figure 6. is
derived from a measurement of the re-circulating Schottky
diode anode current. The re-circulating diode anode should
be connected 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 func-
tion of the Vin and Vout voltages per the following equation:
I
RAMP
= (5µ x (Vin – Vout)) + 25µA
(Continued)
FIGURE 6. Composition of Current Sense Signal
13
Proper selection of the RAMP capacitor depends upon the
selected value of the output inductor. The value of C
can 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 ( 0.5
V / A). The C
the device and connected directly to the pins of the IC
(RAMP and AGND).
For duty cycles greater than 50 percent, current mode con-
trol circuits are subject to sub-harmonic oscillation. Adding a
fixed slope voltage ramp signal (slope compensation) to the
current sense signal prevents this oscillation. The 25µA of
offset current provided from the emulated current source
adds some fixed slope to the ramp signal. In some very high
duty cycle applications, additional slope may be required. In
these applications, the ramp capacitor value can be de-
creased to increase the ramp slope compensation.
RAMP
capacitor should be located very close to
RAMP
= L x 10
-5
, where L is the value
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RAMP