RT8167A
the expense of component size and board space. Figure
11 shows the on-time setting circuit. Connect a resistor
(RTONSETx) between VIN and TONSETx to set the on-time
of UGATEx :
tONx (VREFx
< 1.2V)
=
28 ×10-12 × RTONSETx
VIN − VREFx
(12)
where tONx is the UGATEx turn on period, VIN is the input
voltage of converter, and VREFx is the internal reference
voltage.
When VREFx is larger than 1.2V, the equivalent switching
frequency may be over the maximum design range, making
it unacceptable. Therefore, the VR implements a pseudo-
constant-frequency technology to avoid this disadvantage
of CCRCOT topology. When VREFx is larger than 1.2V,
the on-time equation will be modified to :
tONx (VREFx ≥ 1.2V)
= 23.33 ×10-12 × RTONSETx × VREFx
(13)
VIN − VREFx
On-time translates roughly to switching frequencies. The
on-times guaranteed in the Electrical Characteristics are
influenced by switching delays in external high side
MOSFET. Also, the dead-time effect increases the effective
on-time, reducing the switching frequency. It occurs only
in CCM during dynamic output voltage transitions when
the inductor current reverses at light or negative load
currents. With reversed inductor current, PHASEx goes
high earlier than normal, extending the on-time by a period
equal to the high side MOSFET rising dead time.
For better efficiency of the given load range, the maximum
switching frequency is suggested to be :
fS(MAX) (kHz)
=
tON
−
1
tHS−Delay
×
VREFx(MAX) + ILOAD(MAX) × ⎡⎣RON _ LS−FET + DCR − RDROOP ⎤⎦
VIN(MAX) + ILOAD(MAX) × ⎡⎣RON _ LS−FET − RON _ HS−FET ⎤⎦
(14)
where fS(MAX) is the maximum switching frequency, tHS-
Delay is the turn on delay of high side MOSFET, VREFx(MAX)
is the maximum application DAC voltage of application,
VIN(MAX) is the maximum application input voltage,
ILOAD(MAX) is the maximum load of application, RON_LS-FET
is the low side MOSFET RDS(ON), RON_HS-FET is the high
side MOSFET RDS(ON), DCRL is the inductor DCR, and
RDROOP is the load line setting.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
34
GFX/CORE
VR CCRCOT
TONSETx RTONSETx
R1
VIN
PWM
C1
Generator
VREFx
On-Time
Figure 11. On-Time Setting with RC Filter
Differential Remote Sense Setting
The CORE/GFX VR includes differential, remote-sense
inputs to eliminate the effects of voltage drops along the
PC board traces, CPU internal power routes and socket
contacts. The CPU contains on-die sense pins CORE/
GFX VCC_SENSE and VSS_SENSE. Connect RGNDx to CORE/
GFX VSS_SENSE. Connect FBx to CORE/GFX VCC_SENSE
with a resistor to build the negative input path of the error
amplifier. The precision voltage reference VREFx is referred
to RGND for accurate remote sensing.
Current Sense Setting
The current sense topology of the CORE/GFX VR is
continuous inductor current sensing. Therefore, the
controller can be less noise sensitive. Low offset amplifiers
are used for loop control and over current detection. The
internal current sense amplifier gain (AI) is fixed to be 10.
The ISENxP and ISENxN denote the positive and negative
input of the current sense amplifier.
Users can either use a current sense resistor or the
inductor's DCR for current sensing. Using inductor's DCR
allows higher efficiency as shown in Figure 12. To let
L
DCR
=
RX
× CX
(15)
then the transient performance will be optimum. For
example, choose L = 0.36μH with 1mΩ DCR and
CX = 100nF, to yields for RX :
RX
=
0.36μH
1mΩ ×100nF
=
3.6kΩ
(16)
PHASEx
VCSx
ISENxP
+
AI -
ISENxN
VOUT
(VCORE/VGFX)
L
DCR
RX
CX
CByp
Figure 12. Lossless Inductor Sensing
is a registered trademark of Richtek Technology Corporation.
DS8167A-00 January 2012