L6712
L6712A
TWO-PHASE INTERLEAVED DC/DC CONTROLLER
1
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
Features
2 PHASE OPERATION WITH
SYNCHRONOUS RECTIFIER CONTROL
ULTRA FAST LOAD TRANSIENT RESPONSE
INTEGRATED HIGH CURRENT GATE
DRIVERS: UP TO 2A GATE CURRENT
3 BIT PROGRAMMABLE OUTPUT FROM
0.900V TO 3.300V OR WITH EXTERNAL REF.
±0.9% OUTPUT VOLTAGE ACCURACY
3mA CAPABLE AVAILABLE REFERENCE
INTEGRATED PROGRAMMABLE REMOTE
SENSE AMPLIFIER
PROGRAMMABLE DROOP EFFECT
10% ACTIVE CURRENT SHARING
ACCURACY
DIGITAL 2048 STEP SOFT-START
CROWBAR LATCHED OVERVOLTAGE PROT.
NON-LATCHED UNDERVOLTAGE PROT.
OVERCURRENT PROTECTION REALIZED
USING THE LOWER MOSFET'S RdsON OR A
SENSE RESISTOR
OSCILLATOR EXTERNALLY ADJUSTABLE
AND INTERNALLY FIXED AT 150kHZ
POWER GOOD OUTPUT AND INHIBIT
FUNCTION
PACKAGES: SO-28 & VFQFPN-36
1.1 Applications
■ HIGH CURRENT DC/DC CONVERTERS
■ DISTRIBUTED POWER SUPPLY
2
Description
The device implements a dual-phase step-down controller with a 180 phase-shift between each phase
June 2005
Figure 1. Packages
SO28
VFQFPN-36 (6x6x1.0mm)
Table 1. Order Codes
Package
Tube
L6712D,
SO
L6712AD
L6712Q,
VFQFPN
L6712AQ
Tape & Reel
L6712DTR,
L6712ADTR
L6712QTR,
L6712AQTR
optimized for high current DC/DC applications.
Output voltage can be programmed through the integrated DAC from 0.900V to 3.300V; programming the "111" code, an external reference from
0.800V to 3.300V is used for the regulation.
Programmable Remote Sense Amplifier avoids
use of external resistor divider and recovers losses along distribution line.
The device assures a fast protection against load
over current and Over / Under voltage.An internal
crowbar is provided turning on the low side mosfet
if Over-voltage is detected.
Output current is limited working in Constant Current mode: when Under Voltage is detected, the
device resets, restarting operation.
Rev. 3
1/29
L6712A L6712
Figure 2. Block Diagram
OSC / INH
SGND
VCCDR
VID2
VID1
VID0
PWM1
VCC
LOGIC AND
PROTECTIONS
DAC
VCCDR
CH1
OCP
DIGITAL
SOFT-START
PHASE1
LS
LGATE1
CURRENT
READING
TOTAL
CURRENT
CH2 OCP
UGATE1
ISEN1
PGNDS1
PGND
CURRENT
AVG
CH1 OCP
CURRENT
CORRECTION
PGOOD
HS
LOGIC PWM
ADAPTIVE ANTI
CROSS CONDUCTION
2 PHASE
OSCILLATOR
BOOT1
BAND-GAP
REFERENCE
PGNDS2
CURRENT
READING
CURRENT
CORRECTION
IDROOP
REF_IN/OUT
FBG
LOGIC PWM
ADAPTIVE ANTI
CROSS CONDUCTION
ISEN2
VPROG
CH2
OCP
REMOTE
AMPLIFIER
IFB_START
PWM2
FBR
VSEN
ERROR
AMPLIFIER
DROOP FB
COMP
LS
LGATE2
PHASE2
HS
UGATE2
Vcc
BOOT2
Vcc
Table 2. Absolute Maximum Ratings
Symbol
Parameter
VCC, VCCDR
VBOOT-VPHASE
Value
Unit
To PGND
15
V
Boot Voltage
15
V
15
V
VUGATE1-VPHASE1
VUGATE2-VPHASE2
LGATE1, PHASE1, LGATE2, PHASE2 to PGND
VPHASEx
-0.3 to Vcc+0.3
V
VID0 to VID2
-0.3 to 5
V
All other pins to PGND
-0.3 to 7
V
26
V
±1500
V
±2000
V
Sustainable Peak Voltage. T 35µA).
■ L6712 - Dynamic Maximum Duty Cycle Limitation
The maximum duty cycle is limited as a function of the measured current and, since the oscillator frequency is fixed once programmed, imply a maximum on-time limitation as follow (where T is the switching period T=1/fSW and IOUT is the output current):
⎧
R SENSE
⎪ T = 0.80 ⋅ T I FB = 0µA
T ON,MAX = ( 0.80 – I FB ⋅ 5.73k ) ⋅ T = ⎛ 0.80 – ---------------------- ⋅ I OUT ⋅ 5.73k⎞ ⋅ T = ⎨
⎝
⎠
Rg
⎪ T = 0.40 ⋅ T I FB = 70µA
⎩
This linear dependence has a value at zero load of 0.80·T and at maximum current of 0.40·T typical and
results in two different behaviors of the device:
Figure 9. TON Limited Operation
VOUT
VOUT
0.80·VIN
0.80·VIN
T ON Limited Output
characteristic
0.40·VIN
Resulting Output
characteristic
Desired Output
characteristic and
UVP threshold
0.40·VIN
IOCP=2·IOCPx
(IDROOP=70µA)
a) Maximum output Voltage
IOUT
IOCP=2·IOCPx
(IDROOP=70µA)
IOUT
b) TON Limited Output Voltage
TON Limited Output Voltage.
This happens when the maximum ON time is reached before the current in each phase reaches IOCPx (IIN< 35µA).
FOx
Figure 9a shows the maximum output voltage that the device is able to regulate considering the TON limitation imposed by the previous relationship. If the desired output characteristic crosses the TON limited
maximum output voltage, the output resulting voltage will start to drop after crossing. In this case, the device doesn't perform constant current limitation but only limits the maximum duty cycle following the previous relationship. The output voltage follows the resulting characteristic (dotted in Figure 9b) until UVP is
detected or anyway until IFB = 70µA.
Constant Current Operation
This happens when ON time limitation is reached after the current in each phase reaches IOCPx (IINFOx >
35µA).
The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current
read becomes lower than IOCPx (IINFOx < 35µA) skipping clock cycles. The high side mosfets can be turned
ON with a TON imposed by the control loop at the next available clock cycle and the device works in the
13/29
L6712A L6712
usual way until another OCP event is detected.
This means that the average current delivered can slightly increase also in Over Current condition since
the current ripple increases. In fact, the ON time increases due to the OFF time rise because of the current
has to reach the IOCPx bottom. The worst-case condition is when the ON time reaches its maximum value.
When this happens, the device works in Constant Current and the output voltage decrease as the load
increase. Crossing the UVP threshold causes the device to reset.
Figure 10 shows this working condition.
It can be observed that the peak current (Ipeak) is greater than the IOCPx but it can be determined as follow:
V IN – Vout min
V IN – Vout MIN
I peak = I OCPx + -------------------------------------- ⋅ Ton MAX = I OCPx + --------------------------------------- ⋅ 0.40 ⋅ T
L
L
Where VoutMIN is the minimum output voltage (VID-40% as follow).
The device works in Constant-Current, and the output voltage decreases as the load increase, until the
output voltage reaches the under-voltage threshold (VoutMIN).
The maximum average current during the Constant-Current behavior results:
Ipeak – I OCPx⎞
I MAX,TOT = 2 ⋅ I MAX = 2 ⋅ ⎛ I OCPx + ------------------------------------⎝
⎠
2
In this particular situation, the switching frequency results reduced. The ON time is the maximum allowed
(TonMAX) while the OFF time depends on the application:
Ipeak – I OCPx
T OFF = L ⋅ -------------------------------------V OUt
1
f = ----------------------------------------T ONmax + T OFF
Figure 10. Constant Current operation
Ipeak
Vout
Droop effect
IMAX
IOCPx
TonMAX
UVP
Iout
IMAX,TOT
TonMAX
(IDROOP=50µA)
a) Maximum current for each phase
2·IOCPx (IDROOP=70µA)
b) Output Characteristic
Over current is set anyway when IINFOx reaches 35µA (IFB=70µA). The full load value is only a convention
to work with convenient values for IFB. Since the OCP intervention threshold is fixed, to modify the percentage with respect to the load value, it can be simply considered that, for example, to have on OCP
threshold of 200%, this will correspond to IINFOx = 35µA (IFB = 70µA). The full load current will then correspond to IINFOx = 17.5µA (IFB = 35µA).
Once the UVP threshold has been intercepted, the device resets with all power mosfets turned OFF. Another soft start is then performed allowing the device to recover from OCP once the over load cause has
been removed.
Crossing the UVP threshold causes the device to reset: all mosfets are turned off and a new soft start is
14/29
L6712A L6712
then implemented allowing the device to recover if the over load cause has been removed.
■ L6712A - Fixed Maximum Duty Cycle Limitation
The maximum duty cycle is fixed and constant with the delivered current. The device works in constant
current operation once the OCP threshold has overcome. Refer to the above Constant Current section in
which only the different value in the maximum duty has to be considered as follow:
V IN – Vout min
V IN – Vout MIN
I peak = I OCPx + -------------------------------------- ⋅ Ton MAX = I OCPx + --------------------------------------- ⋅ 0.85 ⋅ T
L
L
All the above reported relationships about the deliverable current once in quasi-constant current and constant current are still valid in this case.
3.5 REMOTE SENSE AMPLIFIER
Remote Sense Amplifier is integrated in order to recover from losses across PCB traces and wiring in high
current DC/DC converter remote sense of the regulated voltage is required to maintain precision in the
regulation. The integrated amplifier is a low-offset error amplifier; external resistors are needed as shown
in Figure 11 to implement a differential remote sense amplifier.
Figure 11. Remote Sense Amplifier Connections
Reference
Reference
ERROR
AMPLIFIER
REMOTE
AMPLIFIER
FBR FBG
R2
VSEN
R2
R1
Remote
VOUT
IDROOP
DROOP
RFB
FB
COMP
CF
ERROR
AMPLIFIER
REMOTE
AMPLIFIER
FBR FBG
IDROOP
VSEN
DROOP
FB
RFB
RF
COMP
CF
RF
R1
Remote
Ground
VOUT
RB used
RB Not Used
Equal resistors give to the resulting amplifier a unity gain: the programmed reference will be regulated
across the remote load.
To regulate output voltages different from the available references, the Remote Amplifier gain can be adjusted simply changing the value of the external resistors as follow (see Figure 11):
V VSEN
RA_Gain = ---------------------------------------------------------------------------------------= R2
-------Remote_V OUT – Remote_GND
R1
to regulate a voltage double of the reference, the above reported gain must be equal to ½.
Modifying the Remote Amplifier Gain (in particular with values higher than 1) allows also to regulate voltages lower than the programmed reference.
Since this Amplifier is connected as a differential amplifier, when calculating the offset introduced
in the regulated output voltage, the "native" offset of the amplifier must be multiplied by the term
KOS = [1+(1/RA_Gain)] because a voltage generator insisting on the non-inverting input represents
the offset.
If remote sense is not required, it is enough connecting RFB directly to the regulated voltage: VSEN becomes not connected and still senses the output voltage through the remote amplifier. In this case the use
of the external resistors R1 and R2 becomes optional and the Remote Sense Amplifier can simply be connected as a "buffer" to keep VSEN at the regulated voltage (See Figure 11). Avoiding use of Remote Amplifier saves its offset in the accuracy calculation but doesn't allow remote sensing.
15/29
L6712A L6712
3.6 INTEGRATED DROOP FUNCTION (Optional)
Droop function realizes dependence between the regulated voltage and the delivered current (Load Regulation). In this way, a part of the drop due to the output capacitor ESR in the load transient is recovered.
As shown in Figure 12, the ESR drop is present in any case, but using the droop function the total deviation of the output voltage is minimized.
Connecting DROOP pin and FB pin together, forces a current IDROOP, proportional to the output current,
into the feedback resistor RFB implementing the load regulation dependence. If RA_Gain is the Remote
Amplifier gain, the Output Characteristic is then given by the following relationship (when droop enabled):
R SENSE
1
1
V OUT = ------------------------- ⋅ ( VID – R FB ⋅ I DROOP ) = ------------------------- ⋅ ⎛ VID – R FB ⋅ ---------------------- ⋅ I OUT⎞
⎠
RA_Gain ⎝
RA_Gain
Rg
with a remote amplifier gain of 1/2, the regulated output voltage results in being doubled.
The Droop current is equal to 50µA at nominal full load and 70µA at the OC intervention threshold, so the
maximum output voltage deviation is equal to:
1
∆V FULL – POSITIVE – LOAD = – ------------------------- ⋅ R ⋅ 50µA
RA_Gain
FB
1
∆V OC – INTERVENTION = – ------------------------- ⋅ R ⋅ 70µA
RA_Gain
FB
Droop function is provided only for positive load; if negative load is applied, and then IINFOx
很抱歉,暂时无法提供与“L6712AQ”相匹配的价格&库存,您可以联系我们找货
免费人工找货