Datasheet

(Typically I
MODE
< 66 mA +
V
IN
160:
)
LM3370
www.ti.com SNVS406N –NOVEMBER 2005–REVISED MAY 2013
PFM Operation
At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency.
The part will automatically transition into PFM mode when either of two conditions are true, for a duration of 32 or
more clock cycles:
1. The NFET current reaches zero.
2. The peak PFET switch current drops below the I
MODE
level .
(3)
Supply current during this PFM mode is less than 20 µA per channel, which allows the part to achieve high
efficiency under extremely light load conditions. When the output drops below the ‘low’ PFM threshold, the cycle
repeats to restore the output voltage to ∼1.2% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see Figure 36) causing the output voltage to fall below the
‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode.
During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage
during PWM operation, allowing additional headroom for voltage 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 0.8% and 1.6% (typical) above the nominal PWM output
voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PFET power switch is turned on.
It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the I
PFM
level
set for PFM mode. The typical peak current in PFM mode is:
I
PFM
= 115 mA + V
IN
/57Ω (4)
Once the PFET power switch is turned off, the NFET power switch is turned on until the inductor current ramps
to zero. When the NFET zero-current condition is detected, the NFET power switch is turned off. If the output
voltage is below the ‘high’ PFM comparator threshold (see Figure 36), the PFET 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 NFET 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 extremely low power mode.
Forced PWM Mode
The LM3370 auto mode can be bypassed by forcing the device to operate in PWM mode, this can be
implemented through the I
2
C-compatible interface, see Table 3.
Soft-Start
The LM3370 has a soft start circuit that limits in-rush current during start up. Soft start is activated only if EN
goes from logic low to logic high after V
IN
reaches 2.7V.
LDO - Low Drop Out Operation
The LM3370 can operate at 100% duty cycle (no switching, PFET switch completely on) for low drop out support
of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.
The minimum input voltage needed to support the output voltage is
V
IN,MIN
= I
LOAD
*(R
DSON,PFET
+ R
INDUCTOR
) + V
OUT
where
• I
LOAD
load current
• • R
DSON/PFET
drain to source resistance of PFET switch in the triode region
• • R
INDUCTOR
inductor resistance (5)
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