LM3687TL-1815EV National Semiconductor, LM3687TL-1815EV Datasheet - Page 14

• Manufacturer Part Number: LM3687TL-1815EV
• Description: BOARD EVALAUTION LM3687TL-1815
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3687TL-1815EV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 2, Non-Isolated
• Voltage - Output: 1.8V, 1.5V
• Current - Output: 750mA, 350mA
• Voltage - Input: 2.7 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1.8MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3687
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

www.national.com
trips at 1172 mA (typ). If the output is shorted to ground the
device enters a timed current limit mode where the NFET is
turned on for a longer duration until the inductor current falls
below a low threshold. This allows the inductor current more
time to decay, thereby preventing runaway.
PFM Operation
At very light load, the DC-DC converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency. The part automatically tran-
sitions into PFM mode when either of two conditions occurs
for a duration of 32 or more clock cycles:
A. The NFET current reaches zero.
B. The peak PMOS switch current drops below the I
el, (typically I
During PFM operation, the DC-DC converter positions the
output voltage slightly higher than the nominal output voltage
FIGURE 4. Typical PFM Operation
MODE
< 36mA + V
BATT
FIGURE 5. Operation in PFM Mode and Transfer to PWM Mode
/ 35Ω ).
20210125
MODE
lev-
14
during PWM operation, allowing additional headroom for volt-
age drop during a load transient from light to heavy load. The
PFM comparators sense the output voltage via the feedback
pin and control the switching of the output FETs such that the
output voltage ramps between ~0.6% and ~1.7% above the
nominal PWM output voltage. If the output voltage is below
the ‘high’ PFM comparator threshold, the PMOS power switch
is turned on. It remains on until the output voltage reaches the
‘high’ PFM threshold or the peak current exceeds the I
level set for PFM mode. The typical peak current in PFM mode
is: I
Once the PMOS power switch is turned off, the NMOS power
switch is turned on until the inductor current ramps to zero.
When the NMOS zero-current condition is detected, the
NMOS power switch is turned off. If the output voltage is be-
low the ‘high’ PFM comparator threshold (see Figure 5), the
PMOS switch is again turned on and the cycle is repeated
until the output reaches the desired level. Once the output
reaches the ‘high’ PFM threshold, the NMOS switch is turned
on briefly to ramp the inductor current to zero and then both
output switches are turned off and the part enters an ex-
tremely low power mode. Quiescent supply current during this
‘sleep’ mode is 27µA (typ), which allows the part to achieve
high efficiency under extremely light load conditions.
If the load current should increase during PFM mode (see
Figure 5) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into fixed-
frequency PWM mode.
When V
mode at ~30mA output current and from PFM to PWM mode
at ~80mA , when V
pens at ~60mA and PFM to PWM transition happens at
~90mA, when V
at ~100mA and PFM to PWM transition happens at ~125mA.
PFM
= 134mA + V
BATT
=2.7V the part transitions from PWM to PFM
BATT
BATT
=5.5V, PWM to PFM transition happens
BATT
=3.6V, PWM to PFM transition hap-
/ 23Ω.
20210126
PFM