LM3676SD-ADJEV National Semiconductor, LM3676SD-ADJEV Datasheet - Page 13

• Manufacturer Part Number: LM3676SD-ADJEV
• Description: BOARD EVALUATION LM3676SD-ADJ
• Manufacturer: National Semiconductor

Specifications of LM3676SD-ADJEV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1 ~ 3.3V
• Current - Output: 600mA
• Voltage - Input: 2.9 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3676
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

SHUTDOWN MODE
Setting the EN input pin low (<0.4V) places the LM3676 in
shutdown mode. During shutdown the PFET switch, NFET
switch, reference, control and bias circuitry of the LM3676 are
turned off. Setting EN high (>1.0V) enables normal operation.
It is recommended to set EN pin low to turn off the LM3676
during system power up and undervoltage conditions when
the supply is less than 2.9V. Do not leave the EN pin floating.
SOFT START
The LM3676 has a soft-start circuit that limits in-rush current
during start-up. During start-up the switch current limit is in-
creased in steps. Soft start is activated only if EN goes from
logic low to logic high after Vin reaches 2.9V. Soft start is im-
plemented by increasing switch current limit in steps of 70mA,
140mA, 280mA and 1020mA (typical switch current limit). The
start-up time thereby depends on the output capacitor and
load current demanded at start-up. Typical start-up times with
a 10µF output capacitor and 300mA load is 400µs and with
1mA load is 275µs.
LDO - LOW DROP OUT OPERATION
The LM3676-ADJ can operate at 100% duty cycle (no switch-
ing; PMOS switch completely on) for low drop out support of
the output voltage. In this way the output voltage will be con-
trolled down to the lowest possible input voltage. When the
device operates near 100% duty cycle, output voltage ripple
is approximately 25 mV.
The minimum input voltage needed to support the output volt-
age is
V
Application Information
OUTPUT VOLTAGE SELECTION FOR LM3676-ADJ
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
to FB, then to GND. V
FB equal to 0.5V. The resistor from FB to GND (R2) should
be 200 kΩ to keep the current drawn through this network well
LM3676-ADJ Configurations For Various V
IN, MIN
• I
• R
• R
V
LOAD
1.875
OUT
DSON, PFET
INDUCTOR
1.1
1.2
1.3
1.5
1.6
1.7
1.8
2.5
2.8
3.3
= I
(V)
LOAD
* (R
DSON, PFET
R1(kΩ)
240
280
320
357
442
432
464
523
402
464
562
OUT
Load current
Drain to source resistance of PFET
switch in the triode region
Inductor resistance
is adjusted to make the voltage at
+ R
INDUCTOR
R2 (kΩ)
200
200
200
178
200
178
178
191
100
100
100
) + V
OUT
C1 (pF)
8.2
8.2
8.2
6.8
8.2
8.2
6.8
15
12
12
10
OUT
13
C2 (pF)
below the 16 µA quiescent current level (PFM mode) but large
enough that it is not susceptible to noise. If R2 is 200 kΩ, and
V
will be 2.5 µA. The output voltage of the adjustable parts
ranges from 1.1V to 3.3V.
The formula for output voltage selection is:
•
•
•
•
For any output voltage greater than or equal to 1.1V, a zero
must be added around 45 kHz for stability. The formula for
calculation of C1 is:
For output voltages higher than 2.5V, a pole must be placed
at 45 kHz as well. If the pole and zero are at the same fre-
quency the formula for calculation of C2 is:
The formula for location of zero and pole frequency created
by adding C1 and C2 is given below. By adding C1, a zero as
well as a higher frequency pole is introduced.
See the "LM3676-ADJ configurations for various V
none
none
none
none
none
none
none
none
none
FB
33
33
V
V
R1: feedback resistor from V
R2: feedback resistor from FB to GND
OUT
is 0.5V, the current through the resistor feedback network
OUT
FB
: feedback voltage = 0.5V
: output voltage (volts)
(Circuit of Figure 2)
L (µH)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
C
IN
OUT
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
(µF)
to FB
C
OUT
www.national.com
OUT
10
10
10
10
10
10
10
10
10
10
10
(µF)
" table.