LM3485MM/NOPB National Semiconductor, LM3485MM/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM3485MM/NOPB
• Description: IC CONTROLLER PFET 8-MSOP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM3485MM/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.24 ~ 35 V
• Current - Output: 4A
• Voltage - Input: 4.5 ~ 35 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Power - Output: 417mW
• Dc To Dc Converter Type: Buck Controller
• Pin Count: 8
• Input Voltage: 4.5 to 35V
• Output Voltage: 1.242 to 35V
• Output Current: 4A
• Package Type: MSOP
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• For Use With: LM3485LED EVAL - BOARD EVALUATION LM3485LEDLM3485EVAL - BOARD EVALUATION LM3485
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Frequency - Switching: -
• Lead Free Status / Rohs Status: Compliant
• Other names: LM3485MM; LM3485MMTR

The minimum output voltage ripple (V
lated in the same way.
For example, with V
Operating frequency (F) is determined by knowing the input
voltage, output voltage, inductor, V
ries Resistance) of output capacitor, and the delay. It can be
approximately calculated using the formula:
where:
α: ( R1 + R2 ) / R2
the PFET delay time. The propagation delay is 90ns
cally. (See the Propagation Delay curve below.)
delay: It includes the LM3485 propagation delay time and
V
OUT_PP
V
FIGURE 1. Hysteretic Window
FIGURE 2. Propagation Delay
OUT_PP
= 0.01* ( 33K + 20K ) / 20K = 0.0266V
OUT
= V
set to 3.3V, V
HYST
( R1 + R2 ) / R2
HYST
OUT_PP
OUT_PP
, ESR (Equivalent Se-
is 26.6mV
) can be calcu-
20034614
20034623
typi-
9
The operating frequency and output ripple voltage can also
be significantly influenced by the speed up capacitor (Cff). Cff
is connected in parallel with the high side feedback resistor,
R1. The location of this capacitor is similar to where a feed
forward capacitor would be located in a PWM control scheme.
However it's effect on hysteretic operation is much different.
The output ripple causes a current to be sourced or sunk
through this capacitor. This current is essentially a square
wave. Since the input to the feedback pin, FB, is a high
impedance node, the current flows through R2. The end result
is a reduction in output ripple and an increase in operating
frequency. When adding Cff, calculate the formula above with
α = 1. The value of Cff depend on the desired operating fre-
quency and the value of R2. A good starting point is 470pF
ceramic at 100kHz decreasing linearly with increased oper-
ating frequency. Also note that as the output voltage is pro-
grammed below 2.5V, the effect of Cff will decrease
significantly.
CURRENT LIMIT OPERATION
The LM3485 has a cycle-by-cycle current limit. Current limit
is sensed across the V
sense resistor. When current limit is activated, the LM3485
turns off the external PFET for a period of 9µs(typical). The
current limit is adjusted by an external resistor, R
The current limit circuit is composed of the ISENSE compara-
tor and the one-shot pulse generator. The positive input of the
ISENSE comparator is the ADJ pin. An internal 5.5µA current
sink creates a voltage across the external R
voltage is compared to the voltage across the PFET or sense
resistor. The ADJ voltage can be calculated as follows:
Where 3.0µA is the minimum I
The negative input of the ISENSE comparator is the ISENSE
pin that should be connected to the drain of the external
PFET. The inductor current is determined by sensing the
V
The current limit is activated when the voltage at the ADJ pin
exceeds the voltage at the I
tor triggers the 9µs one shot pulse generator forcing the driver
to turn the PFET off. The driver turns the PFET back on after
9µs. If the current has not reduced below the set threshold,
the cycle will repeat continuously.
A filter capacitor, C
3. C
DS
. It can be calculated as follows.
ADJ
V
filters unwanted noise so that the ISENSE comparator
ISENSE
FIGURE 3. Current Sensing by V
= V
V
ADJ
IN
ADJ
− (R
= V
, should be placed as shown in
DS
DSON
of the PFET or across an additional
IN
SENSE
− (R
CL-ADJ
* I
ADJ
IND_PEAK
pin. The ISENSE compara-
* 3.0µA)
value.
) = V
ADJ
IN
DS
resistor. This
− V
www.national.com
ADJ
20034625
DS
.
Figure