Datasheet

www.ti.com
GATE
CONTROL
LOGIC
L
1
PGND1
VIN_SM1
SM1
OUT
P1
-
_
+
_
+
_
SET
RESET
OUT
OUTPUT VOLTAGE
COMPARATOR
POWERSTAGEPEAK
CURRENT COMPARATORS
I(L1)
I(L1)
LSM1
BIASCONTROL
PFMCONTROL SECTION
(SHOWNFORSM1,SAMETOPOLOGY FORSM2)
V(VIN_SM1)
39 W
V(VIN_SM1)
29 W
V
O(SM1)
+
3.3 Hm
V
O(SM1)
C21
10 Fm
C22
10 Fm
BURST
V
(OUT)
I
PFM(ENTER)
I
PFM(LEAVE)
BURST
INDUCTOR
CURRENT
OUTPUT
VOLTAGE
LOWRIPPLE
PFMOPERATION
MAXIMUMEFFICIENCY
PFMOPERATION
T
COMP
T
COMP
T
COMP
T
COMP
TPS65810
TPS65811
SLVS658B – MARCH 2006 – REVISED FEBRUARY 2007
Figure 43. PFM Mode Operation
During burst operation two current comparators control the power stage integrated MOSFETs. These
comparators monitor the instantaneous inductor current and compare it to the internal thresholds I
PFM(ENTER)
and
I
PFM(LEAVE)
, turning the p-channel switch on if the inductor current is less than I
PFM(LEAVE)
and turning it off if the
inductor current exceeds I
PFM(ENTER)
. The n-channel switch is turned on when the p-channel MOSFET is off.
The PFM output voltage comparator quiescent current may be reduced using the I
2
C register bits PFM_RPL1
and PFM_RPL2 in registers SM1_SET and SM2_SET. The voltage comparator quiescent current is reduced if
PFM_RPL1 and PFM_RPL2 bits are set to LO, and the comparator response time (t
COMP
, see Figure 44 )
increases. A reduction in quiescent current increases the converter efficiency at light loads, at the expense of a
larger output voltage ripple when in PFM mode.
The ripple is minimized if PFM_RPL1 and PFM_RPL2 bits are set to HI, at the expense of reduced efficiency
under light loads. The operation under low and high ripple settings is described in Figure 44 .
Figure 44. PFM mode operation waveforms
When a burst of pulses is generated, the PFM current comparators control the power-stage MOSFETs to limit
the inductor current to a value between the thresholds I
PFM(LEAVE)
and I
PFM(ENTER)
. The number of pulses in a
burst cycle is proportional to the load current, and the average current is always below I
PFM(LEAVE)
once PFM
operation is set. The typical burst operation in PFM mode is shown in Figure 45 .
Copyright © 2006 – 2007, Texas Instruments Incorporated Submit Documentation Feedback 63
Product Folder Link(s): TPS65810 TPS65811