STMICROELECTRONICS L6910GTR

L6910G
ADJUSTABLE STEP DOWN CONTROLLER
WITH SYNCHRONOUS RECTIFICATION
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Figure 1. Packages
FEATURES
OPERATING SUPPLY VOLTAGE FROM 5V
TO 12V BUSES
UP TO 1.3A GATE CURRENT CAPABILITY
ADJUSTABLE OUTPUT VOLTAGE
N-INVERTING E/A INPUT AVAILABLE
0.9V ±1.5% VOLTAGE REFERENCE
VOLTAGE MODE PWM CONTROL
VERY FAST LOAD TRANSIENT RESPONSE
0% TO 100% DUTY CYCLE
POWER GOOD OUTPUT
OVERVOLTAGE PROTECTION
HICCUP OVERCURRENT PROTECTION
200kHz INTERNAL OSCILLATOR
OSCILLATOR EXTERNALLY ADJUSTABLE
FROM 50kHz TO 1MHz
SOFT START AND INHIBIT
PACKAGE: SO-16
APPLICATIONS
SUPPLY FOR MEMORIES AND TERMINATIONS
COMPUTER ADD-ON CARDS
LOW VOLTAGE DISTRIBUTED DC-DC
MAG-AMP REPLACEMENT
DESCRIPTION
SO-16 (Narrow)
Table 1. Order Codes
Part Number
Package
L6910G
SO-16
L6910GTR
SO-16 in Tape & Reel
dc-dc conversion from 3.3V, 5V and 12V buses.
The output voltage is adjustable down to 0.9V;
higher voltages can be obtained with an external
voltage divider.
High peak current gate drivers provide for fast switching to the external power section, and the output
current can be in excess of 20A.
The device assures protections against load overcurrent and overvoltage.
An internal crowbar is also provided turning on the
low side mosfet as long as the over-voltage is detected. In case of over-current detection, the soft start capacitor is discharged and the system works in
HICCUP mode.
The device is a pwm controller for high performance
Figure 2. Block Diagram
Vin 5V to 12V
VCC
PGOOD
OCSET
MONITOR
PROTECTION
& REF
VREF
BOOT
SS
UGATE
OSC
OSC
RT
E/A
+
300K
EAREF
-
PHASE
PWM
-
VO
LGATE
+
PGND
GND
VFB
COMP
D03IN1509
May 2005
Rev. 1
1/26
L6910G
Table 2. Absolute Maximum Ratings
Symbol
Parameter
Vcc
VBOOT-VPHASE
Value
Unit
Vcc to GND, PGND
15
V
Boot Voltage
15
V
15
V
-0.3 to Vcc+0.3
V
VHGATE-VPHASE
OCSET, LGATE, PHASE
SS, FB, PGOOD, VREF, EAREF, RT
7
V
6.5
V
Junction Temperature Range
-40 to 150
°C
Tstg
Storage temperature range
-40 to 150
°C
Ptot
Maximum power dissipation at Tamb = 25°C
1
W
±1000
V
±2000
V
COMP
Tj
OCSET PIN
OTHER PINS
Maximum Withstanding Voltage Range
Test Condition: CDF-AEC-Q100-002”Human Body Model”
Acceptance Criteria: “Normal Performance”
Table 3. Thermal Data
Symbol
Rth j-amb
Parameter
Thermal Resistance Junction to Ambient
(*) Device soldered on 1 S2P PC board
Figure 3. Pins Connection (Top view)
VREF
1
16
N.C.
OSC
2
15
VCC
OCSET
3
14
LGATE
SS/INH
4
13
PGND
COMP
5
12
BOOT
FB
6
11
HGATE
GND
7
10
PHASE
EAREF
8
9
PGOOD
D03IN1510
2/26
Value
Unit
120
°C/W
L6910G
Table 4. Pins Function
Pin
Name
Description
1
VREF
Internal 0.9V ±1.5% reference is available for external regulators or for the internal error amplifier
(connecting this pin to EAREF) if external reference is not available.
A minimum 1nF capacitor is required.
If the pin is forced to a voltage lower than 70%, the device enters the hiccup mode.
2
OSC
Oscillator switching frequency pin. Connecting an external resistor (RT) from this pin to GND, the
external frequency is increased according to the equation:
6
4.94 ⋅ 10
f OSC,RT = 200KHz + ------------------------R T ( KΩ )
Connecting a resistor (RT) from this pin to Vcc (12V), the switching frequency is reduced according to
the equation:
7
4.306 ⋅ 10
f OSC,RT = 200KHz – ----------------------------R T ( KΩ )
If the pin is not connected, the switching frequency is 200KHz.
The voltage at this pin is fixed at 1.23V. Forcing a 50µA current into this pin, the built in oscillator
stops to switch.
In Over Voltage condition this pin goes over 3V until that conditon is removed.
3
OCSET
A resistor connected from this pin and the upper Mos Drain sets the current limit protection.
The internal 200µA current generator sinks a constant current through the external resistor. The
Over-Current threshold is due to the following equation:
I OCSET ⋅ R OCSET
I P = ---------------------------------------------R DSon
4
SS/INH
The soft start time is programmed connecting an external capacitor from this pin and GND. The
internal current generator forces through the capacitor 10µA.
This pin can be used to disable the device forcing a voltage lower than 0.4V
5
COMP
This pin is connected to the error amplifier output and is used to compensate the voltage control
feedback loop.
6
FB
This pin is connected to the error amplifier inverting input and is used to compensate the voltage
control feedback loop.
Connected to the output resistor divider, if used, or directly to Vout, it manages also over-voltage
conditions and the PGOOD signal
7
GND
8
EAREF
All the internal references are referred to this pin. Connect it to the PCB signal ground.
9
PGOOD This pin is an open collector output and it is pulled low if the output voltage is not within the above
specified thresholds. If not used it may be left floating.
10
PHASE
This pin is connected to the source of the upper mosfet and provides the return path for the high side
driver. This pin monitors the drop across the upper mosfet for the current limit together with OCSET.
11
HGATE
High side gate driver output.
12
BOOT
Bootstrap capacitor pin. Through this pin is supplied the high side driver and the upper mosfet.
Connect through a capacitor to the PHASE pin and through a diode to Vcc (cathode vs. boot).
VBOOT limited to VOCSET -10V(typ.) when all other pins are connected to GND.
13
PGND
Power ground pin. This pin has to be connected closely to the low side mosfet source in order to
reduce the noise injection into the device
14
LGATE
This pin is the lower mosfet gate driver output
15
VCC
Device supply voltage. The operative supply voltage ranges is from 5V to 12V.
DO NOT CONNECT VIN TO A VOLTAGE GREATER THAN VCC.
16
N.C.
This pin is not internally bonded. It may be left floating or connected to GND.
Error amplifier non-inverting input. Connect to this pin an external reference (from 0.9V to 3V) for
the PWM regulation or short it to VREF pin to use the internal reference.
If this pin goes under 650mV (typ), the device shuts down.
3/26
L6910G
Table 5. Electrical Characteristics (Vcc = 12V, TJ =25°C unless otherwise specified)
Symbol
Parameter
Vcc SUPPLY CURRENT
Icc
Vcc Supply current
POWER-ON
Turn-On Vcc threshold
Turn-Off Vcc threshold
Rising VOCSET threshold
Turn On EAREF threshold
SOFT START AND INHIBIT
Iss
Soft start Current
S.S. current in INH condition
OSCILLATOR
Initial Accuracy
fOSC
fOSC,RT
Total Accuracy
∆Vosc
Ramp amplitude
REFERENCE
Output Voltage Accuracy
VOUT
Test Condition
Min
Typ
Max
Unit
4
7
9
mA
4.0
3.8
4.3
4.1
1.24
4.6
4.4
1.4
V
V
V
650
750
mV
6
10
35
14
60
µA
µA
OSC = OPEN
OSC = OPEN; Tj = 0° to 125°
180
170
200
220
230
KHz
kHz
16 KΩ < RT to GND < 200 KΩ
-15
15
%
OSC = open; SS to GND
VOCSET = 4V
VOCSET = 4V
VOCSET = 4V
SS = 2V
SS = 0 to 0.4V
1.9
VOUT = VFB; VEAREF = VREF
0.886
0.900
0.913
0.886
0.900
0.913
V
+2
%
10
µA
300
0.01
0.5
kΩ
µA
VREF
Reference Voltage
CREF = 1nF; IREF = 0 to 100µA
VREF
Reference Voltage
CREF = 1nF; TJ = 0 to 125°C
ERROR AMPLIFIER
IEAREF N.I. bias current
IFB
VCM
VCOMP
GV
EAREF Input Resistance
I.I. bias current
-2
VEAREF = 3V
Vs. GND
VFB = 0V to 3V
0.8
3
V
Output Voltage
0.5
4
V
Open Loop Voltage Gain
70
COMP = 10pF
VBOOT - VPHASE = 12V
VHGATE - VPHASE = 6V
VBOOT - VPHASE = 12V
ILGATE
High Side
Sink Resistance
Low Side Source Current
RLGATE
Low Side Sink Resistance
Vcc = 12V
Output Driver Dead Time
PROTECTIONS
IOCSET OCSET Current Source
Vcc = 12V; VLGATE = 6V
1
85
dB
10
10
MHz
V/µs
1.3
A
4
Ω
3
Ω
210
ns
200
230
µA
117
120
%
2
0.9
1.1
1.5
PHASE connected to GND
90
VOCSET = 4V
170
A
Over Voltage Trip (VFB / VEAREF)
VFB Rising
OSC Sourcing Current
VFB > OVP Trip
15
30
VFB Rising
108
110
112
Lower Threshold (VFB / VEAREF)
VFB Falling
88
90
92
Hysteresis (VFB / VEAREF)
Upper and Lower threshold
VPGOOD
PGOOD Voltage Low
IPGOOD
Output Leakage Current
IOSC
POWER GOOD
Upper Threshold (VFB / VEAREF)
4/26
V
Common Mode Voltage
GBWP Gain-Bandwidth Product
SR
Slew-Rate
GATE DRIVERS
IHGATE High Side
Source Current
RHGATE
V
mA
%
%
2
%
IPGOOD = -4mA
0.4
V
VPGOOD = 6V
0.2
1
µA
L6910G
4
DEVICE DESCRIPTION
The device is an integrated circuit realized in BCD technology. The controller provides complete control logic and protection for a high performance step-down DC-DC converter. It is designed to drive N
Channel Mosfets in a synchronous-rectified buck topology. The output voltage of the converter can be
precisely regulated down to 900mV with a maximum tolerance of ±1.5% when the internal reference is
used (simply connecting together EAREF and VREF pins). The device allows also using an external
reference (0.9V to 3V) for the regulation. The device provides voltage-mode control with fast transient
response. It includes a 200kHz free-running oscillator that is adjustable from 50kHz to 1MHz. The error amplifier features a 10MHz gain-bandwidth product and 10V/µs slew rate that permits to realize
high converter bandwidth for fast transient performance. The PWM duty cycle can range from 0% to
100%. The device protects against over-current conditions entering in HICCUP mode. The device
monitors the current by using the rDS(ON) of the upper MOSFET(s) that eliminates the need for a current sensing resistor. The device is available in SO16 narrow package.
4.1 Oscillator
The switching frequency is internally fixed to 200kHz. The internal oscillator generates the triangular waveform
for the PWM charging and discharging with a constant current an internal capacitor. The current delivered to the
oscillator is typically 50µA (Fsw = 200KHz) and may be varied using an external resistor (RT) connected between
OSC pin and GND or VCC. Since the OSC pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied proportionally to the current sunk (forced) from (into) the pin.
In particular connecting RT vs. GND the frequency is increased (current is sunk from the pin), according to the
following relationship:
6
4.94 ⋅ 10
f OSC,RT = 200KHz + ------------------------R T ( KΩ )
Connecting RT to VCC = 12V or to VCC = 5V the frequency is reduced (current is forced into the pin), according
to the following relationships:
7
4.306 ⋅ 10
f OSC,RT = 200KHz – ----------------------------R T ( KΩ )
VCC = 12V
6
15 ⋅ 10
f OSC,RT = 200KHz – --------------------R T ( KΩ )
VCC = 5V
Switching frequency variation vs. RT are repeated in Fig. 4.
Note that forcing a 50µA current into this pin, the device stops switching because no current is delivered to the
oscillator.
Figure 4.
10000
Resistance [kOhm]
1000
100
10
RT to GND
RT to VCC=12V
RT to VCC=5V
10
100
Frequency [kHz]
1000
5/26
L6910G
4.2 Reference
A precise ±1.5% 0.9V reference is available. This reference must be filtered with 1nF ceramic capacitor to avoid
instability in the internal linear regulator. It is able to deliver up to 100µA and may be used as reference for the
device regulation and also for other devices. If forced under 70% of its nominal value, the device enters in Hiccup mode until this condition is removed.
Through the EAREF pin the reference for the regulation is taken. This pin directly connects the non-inverting
input of the error amplifier. An external reference (or the internal 0.9V ±1.5%) may be used. The input for this
pin can range from 0.9V to 3V. It has an internal pull-down (300kΩ resistor) that forces the device shutdown if
no reference is connected (pin floating). However the device is shut down if the voltage on the EAREF pin is
lower than 650mV (typ).
4.3 Soft Start
At start-up a ramp is generated charging the external capacitor CSS with an internal current generator. The initial
value for this current is of 35µA and speeds-up the charge of the capacitor up to 0.5V. After that it becames
10µA until the final charge value of approximatively 4V.
When the voltage across the soft start capacitor (VSS) reaches 0.5V the lower power MOS is turned on to discharge the output capacitor. As VSS reaches 1.1V (i.e. the oscillator triangular wave inferior limit) also the upper
MOS begins to switch and the output voltage starts to increase.
No switching activity is observable if SS is kept lower than 0.5V and both mosfets are off.
If VCC and OCSET pins are not above their own turn-on thresholds and VEAREF is not above 650mV, the SoftStart will not take place, and the relative pin is internally shorted to GND. During normal operation, if any undervoltage is detected on one of the two supplies, the SS pin is internally shorted to GND and so the SS capacitor
is rapidly discharged.
Figure 5. Soft Start (with Reference Present)
Vcc Turn-on threshold
Vcc
Vin
Vin Turn-on threshold
1V
Vss
to GND
0.5V
LGATE
Vout
Timing Diagram
Acquisition: CH1 = PHASE; CH2 = Vout;
CH3 = PGOOD; CH4 = Vss
4.4 Driver Section
The driver capability on the high and low side drivers allows using different types of power MOS (also multiple
MOS to reduce the RDSON), maintaining fast switching transition.
The low-side mos driver is supplied directly by Vcc while the high-side driver is supplied by the BOOT pin.
Adaptative dead time control is implemented to prevent cross-conduction and allow to use several kinds of mosfets. The upper mos turn-on is avoided if the lower gate is over about 200mV while the lower mos turn-on is
6/26
L6910G
avoided if the PHASE pin is over about 500mV. The lower mos is in any case turned-on after 200ns from the
high side turn-off.
The peak current is shown for both the upper (fig. 6) and the lower (fig. 7) driver at 5V and 12V. A 3.3nF capacitive load has been used in these measurements.
For the lower driver, the source peak current is 1.1A @ VCC = 12V and 500mA @ VCC = 5V, and the sink peak
current is 1.3A @ VCC = 12V and 500mA @ VCC = 5V.
Similarly, for the upper driver, the source peak current is 1.3A @ Vboot-Vphase = 12V and 600mA @ VbootVphase = 5V, and the sink peak current is 1.3A @ Vboot-Vphase =12V and 550mA @ Vboot-Vphase = 5V.
Figure 6. High Side Driver Peak Current. Vboot-Vphase = 12V (right) Vboot-Vphase = 5V (left)
CH1 = High Side Gate CH4 = Gate Current
Figure 7. Low Side Driver Peak Current. VCC = 12V (right) VCC = 5V (left)
CH1 = Low Side Gate CH4 = Gate Current
4.5 Monitoring and Protections
The output voltage is monitored by means of pin FB. If it is not within ±10% (typ.) of the programmed value, the
powergood output is forced low.
The device provides overvoltage protection, when the voltage sensed on pin FB reaches a value 17% (typ.)
greater than the reference the OSC pin is forced high (3V typ.) and the lower driver is turned on as long as the
over-voltage is detected.
7/26
L6910G
Overcurrent protection is performed by the device comparing the drop across the high side MOS, due to the
RDSON, with the voltage across the external resistor (ROCS) connected between the OCSET pin and drain of the
upper MOS. Thus the overcurrent threshold (IP) can be calculated with the following relationship:
R OCS ⋅ I OCS
I P = --------------------------------R dsON
Where the typical value of IOCS is 200µA. To calculate the ROCS value it must be considered the maximum
RdsON (also the variation with temperature) and the minimum value of IOCS. To avoid undesirable trigger of
overcurrent protection this relationship must be satisfied:
∆I
I P ≥ I OUTMAX + ----- = I PEAK
2
Where ∆I is the inductance ripple current and IOUTMAX is the maximum output current.
In case of over current detectionthe soft start capacitor is discharged with constant current (10µA typ.) and when
the SS pin reaches 0.5V the soft start phase is restarted. During the soft start the over-current protection is always active and if such kind of event occurs, the device turns off both mosfets, and the SS capacitor is discharged again (after reaching the upper threshold of about 4V). The system is now working in HICCUP mode,
as shown in figure 8. After removing the cause of the over-current, the device restart working normally without
power supplies turn off and on.
Figure 8. Hiccup Mode
Figure 9. Inductor Ripple Current vs. Vout
9
L=1.5µH, Vin=12V
Inductor Ripple [A]
8
7
L=2µH,
Vin=12V
6
L=3µH,
Vin=12V
5
4
L=1.5µH,
Vin=5V
3
L=2µH,
Vin=5V
2
L=3µH, Vin=5V
1
0
0.5
CH1 = SS; CH4 = Inductor current
1.5
2.5
3 .5
Output V oltage [V ]
4.6 Inductor Design
The inductance value is defined by a compromise between the transient response time, the efficiency, the cost
and the size. The inductor has to be calculated to sustain the output and the input voltage variation to maintain
the ripple current ∆IL between 20% and 30% of the maximum output current. The inductance value can be calculated with this relationship:
V IN – V OUT V OUT
L = ------------------------------ ⋅ --------------f sw ⋅ ∆I L
V IN
Where fSW is the switching frequency, VIN is the input voltage and VOUT is the output voltage. Figure 9 shows
the ripple current vs. the output voltage for different values of the inductor, with VIN = 5V and VIN = 12V.
Increasing the value of the inductance reduces the ripple current but, at the same time, reduces the converter
response time to a load transient. If the compensation network is well designed, the device is able to open or
close the duty cycle up to 100% or down to 0%. The response time is now the time required by the inductor to
change its current from initial to final value. Since the inductor has not finished its charging time, the output current is supplied by the output capacitors. Minimizing the response time can minimize the output capacitance
required.
8/26
L6910G
The response time to a load transient is different for the application or the removal of the load: if during the application of the load the inductor is charged by a voltage equal to the difference between the input and the output
voltage, during the removal it is discharged only by the output voltage. The following expressions give approximate response time for ∆I load transient in case of enough fast compensation network response:
L ⋅ ∆I
t application = -----------------------------V IN – V OUT
L ⋅ ∆I
t removal = --------------V OUT
The worst condition depends on the input voltage available and the output voltage selected. Anyway the worst
case is the response time after removal of the load with the minimum output voltage programmed and the maximum input voltage available.
4.7 Output Capacitor
The output capacitor is a basic component for the fast response of the power supply. In fact, during load transient, for first few microseconds they supply the current to the load. The controller recognizes immediately the
load transient and sets the duty cycle at 100%, but the current slope is limited by the inductor value. The output
voltage has a first drop due to the current variation inside the capacitor (neglecting the effect of the ESL):
∆V OUT = ∆I OUT ⋅ ESR
A minimum capacitor value is required to sustain the current during the load transient without discharge it. The
voltage drop due to the output capacitor discharge is given by the following equation:
2
∆I OUT ⋅ L
∆V OUT = --------------------------------------------------------------------------------------------2 ⋅ C OUT ⋅ ( V INMIN ⋅ D MAX – V OUT )
Where DMAX is the maximum duty cycle value that is 100%. The lower is the ESR, the lower is the output drop
during load transient and the lower is the output voltage static ripple.
4.8 Input Capacitor
The input capacitor has to sustain the ripple current produced during the on time of the upper MOS, so it must
have a low ESR to minimize the losses. The rms value of this ripple is:
I rms = I OUT D ⋅ ( 1 – D )
Where D is the duty cycle. The equation reaches its maximum value with D = 0.5. The losses in worst case are:
2
P = ESR ⋅ I rms
4.9 Compensation Network Design
The control loop is a voltage mode (figure 10). The output voltage is regulated to the input Reference voltage
level (EAREF). The error amplifier output VCOMP is then compared with the oscillator triangular wave to provide
a pulse-width modulated (PWM) wave with an amplitude of VIN at the PHASE node. This wave is filtered by the
output filter. The modulator transfer function is the small-signal transfer function of VOUT/VCOMP. This function
has a double pole at frequency FLC depending on the L-Cout resonance and a zero at FESR depending on the
output capacitor ESR. The DC Gain of the modulator is simply the input voltage VIN divided by the peak-to-peak
oscillator voltage ∆VOSC.
9/26
L6910G
Figure 10. Compensation Network
VIN
∆VOSC
L
VOUT
ESR
PWM
COMPARATOR
COUT
C18
R5
C19
R3
C20
VCOMP
R4
EAREF
-
D03IN1512
+
The compensation network consists in the internal error amplifier and the impedance networks ZIN (R3, R4 and
C20) and ZFB (R5, C18 and C19). The compensation network has to provide a closed loop transfer function with
the highest 0dB crossing frequency to have fast response (but always lower than fsw/10) and the highest gain
in DC conditions to minimize the load regulation.
A stable control loop has a gain crossing with -20dB/decade slope and a phase margin greater than 45°. Include
worst-case component variations when determining phase margin.
To locate poles and zeroes of the compensation networks, the following suggestions may be used:
Modulator singularity frequencies:
1
ω LC = --------------------------L ⋅ C OUT
1
ω ESR = --------------------------------ESR ⋅ C OUT
Compensation network singularity frequency:
1
ω P1 = ----------------------------------------------C18 ⋅ C19
R5 ⋅  -----------------------------
 C18 + C19
1
ω Z1 = -----------------------R5 ⋅ C19
1
ω P2 = -----------------------R4 ⋅ C20
1
ω Z2 = ------------------------------------------( R3 + R4 ) ⋅ C20
– Put the gain R5/R3 in order to obtain the desired converter bandwidth;
– Place ωZ1 before the output filter resonance ωLC;
– Place ωZ2 at the output filter resonance ωLC;
– Place ωP1 at the output capacitor ESR zero ωESR;
– Place ωP2 at one half of the switching frequency;
– Check the loop gain considering the error amplifier open loop gain.
10/26
L6910G
Figure 11. Asymptotic Bode Plot of Converter's Gain
dB
Error Amplifier
R5/R3
ωΖ1
ωLC
ωΖ2
ωP1
ωP2
ω
ωESR
Modulator Gain
Compensation Network Gain
Error Amplifier
5
Closed Loop Gain
15A DEMO BOARD DESCRIPTION
The demo board shows the operation of the device in a general purpose application. This evaluation board allows voltage adjustability from 0.9V to 5V through the switches S2-S5 according to the reported table when the
internal 0.9V reference is used (G1 closed). Output current in excess of 20A can be reached dependently on
the kind of mosfet used: up to three SO8 mosfet may be used for both High side and Low side switches. External
reference may be used for the regulation simply leaving open G1 and the switches S2-S5. The device may also
be disabled with the switch S1. The VCC input rail supplies the device while the power conversion starts from
the VIN input rail. The device is also able to operate with a single supply voltage; in this case the jumper G2 has
to be closed and a 5V to 12V input can be directly connected to the VIN input. The four layers demo board's
copper thickness is of 70µm in order to minimize conduction losses considering the high current that the circuit
is able to deliver.The PGOOD signal is used as a logic level and it's been pulled up to VIN because there's no
other appropriate voltage available on the demo board. In case of input voltage higher than 7V (PGOOD Pin
Maximum Absolute Rating) a 5V reference is required. Figure 12 shows the demo board's schematic circuit
Figure 12. 15A Demo Board Schematic
L1
F1
VIN
GNDIN
R7
G2
C1-C3
C14
C13
D1
VCC
R6
VCC
C17
GNDCC
OCSET
12
Q1-3
3
11
C15
GND
EAREF
REFIN
C16
GNDREFIN
BOOT
15
10
7
14
8
13
UGATE
L2
PHASE
LGATE
C411
D2
R2
PGND
GNDOUT
G1
VREF
+VREF
9
1
PGOOD
PWRGD
R8
C12
GNDREF
VOUT
Q4-6
R9
SS
C21
OSC
4
2
R1
C22
5
COMP
6
C19
VFB
R5
VOUT
R3
0.9
R4
C20
S2
S3
S4
S5
Open Open Open Open
1.2
ON Open Open Open
R10
1.5
Open ON Open Open
S3
R11
1.8
S4
SR12
2.5
Open Open ON Open
S5
SR13
3.3
Open Open Open ON
5.0
Open Open ON
C18
S1
S2
D03IN1513
ON
ON Open Open
ON
11/26
L6910G
Table 6. Part List
Reference
Description
Manufacturer
R1
N.C
NEOHM
R2
10K
5% 125mW
NEOHM
SMD 0805
R3
4.7K
5% 125mW
NEOHM
SMD 0805
R4
1KOhm 5% 125mW
NEOHM
SMD 0805
R5
2.7K
5% 125mW
NEOHM
SMD 0805
R6
10Ohm 5% 125mW
NEOHM
SMD 0805
R7
510Ohm 5% 125mW
NEOHM
SMD 0805
R8
N.C
R9
0 Ohm
SMD 0805
R10
14K
5% 125mW
NEOHM
SMD 0805
R11
6.98K
5% 125mW
NEOHM
SMD 0805
R12
2.61K
5% 125mW
NEOHM
SMD 0805
R13
1.74K
5 5% 125mW
NEOHM
SMD 0805
RADIAL 10X10.5
C1, C3
100µF - 20V
OSCON 20SA100M
C9, C10
330µF - 6.3V
POSCAP 6TPB330M
SMD7343
C12, C13,
C15, C21
100nF
KEMET
SMD0805
SMD0805
C14
1nF
KEMET
C16
100nF
KEMET
C17
4.7µF - 16V
AUX
SMA6032
C18
1.5nF
KEMET
SMD0805
C19
15nF
KEMET
SMD0805
C20
47nF
KEMET
SMD0805
C22
N.C
L1
Short
L2
3µH (T50-52B Core, 7T AWG15)
MICROMETALS
Q2,Q3,Q4,Q6
STS11NF30L
ST
D1
1N4148
D2
STPS2L25U
ST
SMB
U1
Device L6910G
ST
SO16Narrow
F1
Short
SWITCH
DIP SWITCH 6 POS.
SO8
SOT23
Table 7. Other Inductor Manufacturer
Manufacturer
WÜRTH ELEKTRONIK
PANASONIC
SUMIDA
12/26
Series
Inductor Value (µH)
Saturation Current (A)
744318
1.8 to 2.7
16 to 20
ETQP6F1R8FA
1.8
20
CDEP134-2R7MC-H
2.7
15
L6910G
Figure 13. PCB and Components Layouts
Component Side
Internal Signal GND Layer
Figure 14. PCB and Components Layouts
Internal Power GND Layer
Solder Side
Figure 15. Efficiency vs Output Current
100
Efficiency (%)
95
Vo=3.3V
90
Vo=2.5V
Vo=1.8V
85
Vo=1.5V
Vo=1.2V
Vin=Vcc=5V
Fsw=200KHz
80
Vo=0.9V
75
1
3
5
7
9
11
13
15
17
Output Current (A)
13/26
L6910G
Figure 16. Efficiency vs Output Current
100
Efficiency (%)
95
Vo=5V
90
Vo=3.3V
85
Vo=1.8V
80
Vo=2.5V
Vo=1.2V
75
Vo=0.9V
Vo=1.5V
70
65
Vin=Vcc=12V
Fsw.=200KHZ
60
55
50
1
3
7Output Current
9
(A)
5
11
13
15
17
Output Current (A)
6
COMPONENTS SELECTION
6.1 Inductor Selection
To select the right inductor value, the application conditions must be fixed. For example we can consider:
Vin=12V Vout =3.3V Iout=15A
Considering a ripple of approximately 25% to 30% of Iout, the inductor value will be L=3 µH.
An iron powder core (TO50-52B) with 7 windings has been chosen.
6.2 Output Capacitors
2 POSCAP capacitors, model 6TPB330M, have been chosen, with a maximum ERS equal to 40mΩ each.
Therefore, the resultant ESR is of 20mΩ. Considering a current ripple of 4A, the output voltage ripple is:
∆Vout = 4 · 0.02 = 80mV
6.3 Input Capacitors
For IOUT = 15A and D = 0.5 (worst case for input current ripple), the RMS current of the input capacitor is equal
to 7.5A.
Two OSCON electrolytic capacitors 6SP680M, with a maximum ESR equal to 13mΩ, have been chosen to sustain the ripple. Therefore, the resultant ESR is equal to 13mΩ/2 = 6.5mΩ. The losses, in worst case, are:
P = ESR · I2rms = 366mW
6.4 Over-Current Protection
The current limit can be set to approximately 20A. Substituting the demo board parameters in the relationship reported in the relative section, (IOSCMIN =170µA; IP = 20A; RDSONMAX = 9mΩ / 2=4.5mΩ) it results
that ROCS = 510Ω
14/26
L6910G
6.5 APPLICATION SUGGESTIONS FOR HIGHER CURRENTS
For higher output currents, up to 20A, the following configuration can be used (with reference to the demo board
schematic):
Q1,Q2,Q3: STS11NF30L
Q4,Q5,Q6: STS17NF3LL
L: 2.5µH Magnetic 77121A7 Core 7T 2x AWG16
In these conditions, the following performance have been achieved:
Table 8.
VIN (V)
VOUT (V)
IOUT (A)
η (%)
VIN (V)
VOUT (V)
IOUT (A)
η (%)
5
1.2
20
81
12
1.2
20
80
5
1.5
20
83
12
1.5
20
83
5
1.8
20
85
12
1.8
20
85
5
2.5
20
89
12
2.5
20
88
5
3.3
20
91
12
3.3
20
91
12
5
20
93
For currents higher than 20A, bigger mosfets should be selected (e.g. STS25NH3LL) both for the high side and
low side (depending on the duty cycle and input voltage).
7
6A DEMO BOARD DESCRIPTION
A compact demo board has been realized to manage currents in the range of 5A-6A .
The external power mosfets are included in a single SO8 package to save space and increase power density.
Two separate rails are provided, for VCC and VIN. They can be connected together by shorting the jumper J1.
The PGOOD signal is used as a logic level and it's been pulled up to VIN because there's no other appropriate
voltage available on the demo board. In case of input voltage higher than 7V (PGOOD Pin Maximum Absolute Rating) a 5V reference is required.
Figure 17. 6A Demo Board Schematic
VIN
R7
J1
GNDIN
C7
D1
R6
VCC
VCC
3
11
C5
GNDCC
10
GND
14
U1
L6910
4
OSC
13
2
EAREF
9
8
C8
1
5
R10
UGATE
C1- C2
R8
LGATE
PGND
L1
Q1/Q1
PHASE
7
SS
C9
C6
OCSET
BOOT
12
15
VOUT
R9
R11
Q2/Q1
D2
R2
C10
C3-4
GNDOUT
PGOOD
PWRGD
VREF
6
VFB
COMP
R3
R5
C19
R4
C20
C18
R1
15/26
L6910G
Table 9. Part List
Reference
Description
Manufacturer
2K7 Ohm 0805 5% 125mW
NEOHM
(Vout = 2.5V)
1K8 Ohm 0805 5% 125mW
NEOHM
(Vout = 3.3V)
1K Ohm 0805 5% 125mW
NEOHM
(Vout = 5V)
R2
10K 5% 125mW
NEOHM
SMD 0805
R3
4K7 5% 125mW
NEOHM
SMD 0805
R4
4K7 5% 125mW
NEOHM
SMD 0805
R5
2K7 5% 125mW
NEOHM
SMD 0805
R6
10 Ohm 5% 125mW
NEOHM
SMD 0805
Resistor
R1
R7
680 Ohm 5% 125mW
NEOHM
SMD 0805
R8 R9
2.2 Ohm 5% 125mW
NEOHM
SMD 0805
R10
N.C
R11
N.C
C34Y5U1E106ZTE12
Capacitors
C1,C2
10µF 25V
TOKIN
C3,C4
100µF - 6.3V
POSCAP 6TPB100M
SMD7343
C5,C6,C9
100nF
KEMET
SMD0805
C7,C8
1nF
KEMET
SMD0805
C10
N.C
C18
1.5nF
KEMET
SMD0805
C19
15nF
KEMET
SMD0805
C20
47nF
KEMET
SMD0805
Magnetics
L1
7µH (T50-52B Core, 12T AWG 21)
MICROMETALS
Transistor
Q1
STS8DNF3LL
ST
Diodes
D1
1N4148
D2
STPS2L25U
ST
SOT23
SMB
Device L6910G
ST
SO16Narrow
Device
U1
Table 10. Other inductor manufacturer
Manufacturer
Series
Inductor Value (µH)
Saturation Current (A)
WÜRTH ELEKTRONIK
744 382
4.8 to 5.8
7.5 to 8
PANASONIC
ETQP6F
4.6 to 6.4
9.3 to 7.9
CDEP134-H
6 to 8
7.2 to 9.6
DO3316P-472HC
4.7
5.4
DO3340P
10 to 22
8 to 5.5
DR125-8R2
8.2
7.8
SUMIDA
COILCRAFT
COILTRONICS
16/26
L6910G
Figure 18. PCB and Components Layouts
Component Side
Solder Side
7.1 Compact Demo Board Performances
Figures 19, 20 show the measured efficiency versus load current for different values of output voltage. The measure has been done at 5V and 12V input. Output voltage has been changed modifying the value of R1 in the
demo board as reported in the part list.
Figure 19. Efficiency vs. Output Current
100
Efficiency (%)
95
90
Vo=3.3V
Vo=2.5V
85
Vo=1.8V
80
Vo=1.5V
75
Vin=Vcc=5V
Fsw=200KHz
Vo=1.2V
70
1
2
3
4
5
6
7
8
Output Current (A)
Figure 20. Efficiency vs. Output Current
95
Vo=5V
Efficiency (%)
90
Vo=3.3V
85
Vo=2.5V
80
Vo=1.8V
Vo=1.5V
Vin=Vcc=12V
Fsw=200KHz
75
Vo=1.2V
70
1
2
3
4
5
6
7
8
Output Current (A)
17/26
L6910G
8
APPLICATION IDEA 1: DDR MEMORY AND TERMINATION SUPPLY
Double Data Rate (DDR) Memories require a particular Power Management Architecture. This is due to the fact
that the trace between the driving chipset and the memory input must be terminated with resistors.
Since the Chipset driving the Memory has a push pull output buffer, the Termination voltage must be capable
of sourcing and sinking current.
Moreover, the Termination voltage must be equal to one half of the memory supply (the input of the memory is
a differential stage requiring a reference bias midpoint) and in tracking with it. For DDRI the Memory Supply is
2.5V and the Termination voltage is 1.25V while, for DDRII, the Memory Supply is 1.8V and the Termination
voltage is 0.9V. Fig. 23 shows a complete DDRI Memory and Termination Power Supply realized by using 2 x
L6910G. The 2.5V section is powering the memory while the 1.25V section is providing the termination voltage.
The tracking between the two sections is realized by providing the EAREF voltage of the 1.25V section through
a resistor divider connected to the 2.5V.
Figure 21. Application idea : DDR Memory Supply
VIN
12V
BOOT
OCSET
12
15
VCC
3
UGATE
11
STS11NF3LL
GND
7
U1
14
L6910
13
DDR
MEMORY
PHASE
10
VDDQ
LGATE
VREF
STS11NF3LL
SS
4
OSC
2
EAREF
PGOOD
9
8
PWRGD
VREF
1
5
[email protected]
PGND
TERMINATION
NETWORK
6
VFB
BUS
COMP
VIN
12V
BOOT
VCC
11
10
GND
4
OSC
U2
14
L6910
13
2
EAREF
9
8
1
5
CHIPSET
PHASE
LGATE
PGND
PGOOD
VTT
[email protected] +- 5A
PWRGD
VREF
6
VFB
COMP
R
UGATE
7
SS
R
STS8DNF3LL
OCSET
3
12
15
+
The current required by the memory and the termination supply, depends on the memory type and size.
The figure 22, 23 shows the efficiency of the L6910G for the termination section of the application shown in fig.
21, in sink and source mode. The figures show the efficiency values also when the input voltage is coming directly from the 12V rail.
18/26
L6910G
Figure 22. Efficiency vs Output Current Source
Mode
Figure 25. Efficiency vs Output Current Sink
Mode
95
100
90
Vin=2.5V
85
Efficiency (%)
Efficiency (%)
90
80
75
Vin=12V
Vcc=12V
Vout=1.25V
Fsw=200KHz
70
65
Vin=1.8V
80
70
Vin=12V
60
Vcc=12V
Vout=0.9V
Fsw=200KHz
50
60
40
1
2
3
4
5
6
7
1
8
Output Current (A)
3
5
7
9
11
13
15
17
Output Current (A)
Figure 23. Efficiency vs Output Current Sink
Mode
Figure 26. Efficiency vs Output Current Source
Mode
95
100
85
Vout=2.5V
90
Vin=2.5V
80
Efficiency (%)
Efficiency (%)
90
75
70
Vcc=12V
Vout=1.25V
Fsw=200KHz
65
Vin=12V
80
Vout=12V
70
Vcc=12V
Vout=1.25V
Fsw=200KHz
60
60
1
2
3
4
5
6
7
8
50
Output Current (A)
1
Figure 24. Efficiency vs Output Current Sink
Mode
100
Efficiency (%)
90
80
Vin=12V
9
11
13
15
17
100
90
Vin=1.8V
80
Vin=12V
70
Vin=12V
Vout=0.9V
Fsw=200KHz
50
Vin=12V
1
70
3
5
7
9
11
13
15
17
Output Current (A)
Vcc=12V
Vout=1.25V
Fsw=200KHz
60
7
Figure 27. Efficiency vs Output Current Source
Mode
60
Vin=2.5V
5
Output Current (A)
Efficiency (%)
For very big systems (e.g. servers), the DDR memory
termination can require much higher currents, in the
range of 10A-15A and more.
Figures 24, 25 and 26, 27 show the efficiency of the
L6910G in sink and source mode, up to 17A both for
DDRI and DDRII memories.The measurements have
been realized with the 15A demo board. (See pag.11 )
3
50
1
3
5
7
9
11
13
Output Current (A)
15
17
19/26
L6910G
9
APPLICATION IDEA 2: POSITIVE BUCK-BOOST REGULATOR 3V TO 13.2V
INPUT / 5V 2.5A OUTPUT
In some applications the input voltage changes in a very wide range while the output must be regulated to a
fixed value. In this case a Buck-Boost topology can be required in order to keep the output voltage in regulation.
The schematic below shows how to implement a Buck-Boost regulating 5V at the output from both 3.3V and 5V
and 12V input buses.
In a Buck-Boost topology the current is delivered to the output during the OFF phase only. So, for a given current
limit, the maximum output current depends strongly on the duty cycle. Assuming a 100% efficiency and neglecting the current ripple across the inductor, the relationship betweent the current limit and the maximum output
current is the following:
I OMAX = I LIM ⋅ ( 1 – D )
Where ILIM is the current limit and D is the duty cycle of the application.
The worst case is with DMAX. Since, in a Buck-Boost application, D is given by the following formula:
VO
D = ----------------------V IN + V O
The worst case is with VINMIN.
Obviously, since the efficiency is lower than 100% and the ripple is usually not negligible, the maximum output
current is always lower than the value calculated in the above formula
Figure 28. Positive buck-boost regulator 3V to 13.2V input / 5V 2.5A Output Circuit
VIN (3.3V-5V-12V BUSES)
C3
R1
G1
GNDIN
D1
C4
BOOT
R7
VCC (12V BUS)
C6
VCC
3
11
10
GND
SS
OSC
EAREF
UGATE
PHASE
Q4
L1
Q1
VOUT ( 5V 2.5A )
7
4
U1
L6910/A
2
8
C7
14
13
9
1
5
6
LGATE
R2
PGND
D2
Q2
C10
R6
Q3
-14
C13-14
C13
GNDOUT
PGOOD
VREF
C12
VFB
COMP
R5
C9
20/26
C1- C2
OCSET
C5
GNDCC
C8
12
15
R3
R4
C11
L6910G
Table 11. Part List
Reference
Description
Manufacturer
R1
910 Ohm 5% 125mW
NEOHM
SMD 0805
R2
10K 5% 125mW
NEOHM
SMD 0805
R3
4.7K 5% 125mW
NEOHM
SMD 0805
R4
1K 5% 125mW
NEOHM
SMD 0805
R5
2.7K 5% 125mW
NEOHM
SMD 0805
R6
1K1
NEOHM
SMD 0805
R7
10 Ohm 125mW
NEOHM
SMD 0805
C1,C2
100µF - 20V
OSCON 20SA100M
RADIAL 10X10.5
C13,C14
330µF - 6.3V
POSCAP 6TPB330M
SMD7343
C12,C5,C8
100nF
KEMET
SMD0805
C3
1nF
KEMET
SMD0805
C4
470nF
KEMET
SMD0805
C6
4.7µF - 16V
AUX
SMA6032
C7
100nF
KEMET
C9
15nF
KEMET
SMD0805
C10
1.5nF
KEMET
SMD0805
C11
47nF
KEMET
SMD0805
G1
Open
Jumper
L1
2.5µH (77121A7 Core, Double winding 7 AWG16)
MAGNETICS
Q1,Q2,Q3
STS11NF30L
ST
SO8
Q4
STS5P30L
ST
SO8
D1
1N4148
D2
STPS3L25U (STPS340U)
ST
SMB (D0144)
U1
Device L6910G
ST
SO16 Narrow
SOT23
Figure 29. Efficiency vs. Output Current
90
Vin=5V
Efficiency (%)
85
80
Vin=12V
75
Vin=3.3V
Vcc=5V
Vout=5V
Fsw=200KHz
70
65
1
1.5
2
2.5
3
3.5
Output Current (A)
21/26
L6910G
10 APPLICATION IDEA 3: BUCK-BOOST REGULATOR 3V TO 5.5V INPUT/-5V
3A OUTPUT
In applications where a negative output voltage is required, a standard Buck-Boost topology can be implemented. The considerations related to the maximum output current are the same of the "Positive Buck-Boost" (Application Idea 2).
A particularity of this topology is that the device undergoes a voltage that is the sum of VIN and VOUT. So, converting 5V to -5V, the device undergoes 10V voltage. It must be checked that the sum of the input and output
voltage is lower than the maximum operating input voltage of the device.
Figure 30. buck-boost regulator 3V to 5.5V input / -5V 3A Output Circuit
VIN (3V to 5.5V )
R1
G1
C3
C1- C2
GNDIN=GNDOUT
D1
C4
BOOT
R7
VCC
VCC (5V)
15
OCSET
3
12
UGATE
10
PHASE
14
LGATE
13
PGND
L1
Q1
C5
C6
GND
GNDCC
11
GNDOUT
7
SS
4
OSC
U1
L6910/A
2
EAREF
8
C7
5
C8
6
9
PGOOD
1
VREF
C13 14
-
VOUT (-5V 3A)
C12
VFB
COMP
R5
C9
D2
Q2
R3
R4
C11
C10
R6
Table 12. Part List
Reference
Description
Manufacturer
R1
910 Ohm 5% 125mW
NEOHM
SMD 0805
R2
10K 5% 125mW
NEOHM
SMD 0805
R3
4.7K 5% 125mW
NEOHM
SMD 0805
R4
1K Ohm 5% 125mW
NEOHM
SMD 0805
R5
2.7K 5% 125mW
NEOHM
SMD 0805
R6
1K 5% 125mW
NEOHM
SMD 0805
R7
10 Ohm 5% 125mW
NEOHM
SMD 0805
C1,C2
100µF - 20V
OSCON 20SA100M
RADIAL 10X10.5
C13,C14
330µF - 6.3V
POSCAP 6TPB330M
SMD7343
C12,C4,C5,C8
100nF
KEMET
SMD0805
C3
1nF
KEMET
SMD0805
C6
4.7µF - 16V
AUX
SMA6032
C7
100nF
KEMET
22/26
L6910G
Table 12. Part List (continued)
C9
15nF
KEMET
SMD0805
C10
1.5nF
KEMET
SMD0805
Reference
Description
Manufacturer
C11
47nF
KEMET
G1
Open
Jumper
L1
2.5µH (77121A7 Core, Double winding 7 AWG16)
MAGNETICS
Q1,Q2
STS11NF30L
ST
SO8
SMD0805
D1
1N4148
D2
STPS3L25U ( STPS340U)
ST
SMB (D0144)
SOT23
U1
Device L6910G
ST
SO16 Narrow
Figure 31. Efficiency vs. Output Current
94
92
Efficiency (%)
Vin=5V
90
88
Vin=3.3V
86
Vcc=5V
Vout= -5V
Fsw=200KHz
84
82
1
1.5
2
2.5
3
Output Current (A)
23/26
L6910G
Figure 32. SO-16 (Narrow) Mechanical Data & Package Dimensions
mm
inch
DIM.
MIN.
TYP.
A
a1
MAX.
MIN.
TYP.
1.75
0.1
0.25
a2
MAX.
0.069
0.004
0.009
1.6
0.063
b
0.35
0.46
0.014
0.018
b1
0.19
0.25
0.007
0.010
C
0.5
c1
D(1)
E
(typ.)
9.8
10
0.386
0.394
5.8
6.2
0.228
0.244
1.27
e3
F
(1)
0.020
45°
e
0.050
8.89
3.8
0.350
4.0
0.150
0.157
G
4.60
5.30
0.181
0.208
L
0.4
1.27
0.150
0.050
M
S
OUTLINE AND
MECHANICAL DATA
0.62
0.024
8 ° (max.)
SO16 (Narrow)
(1) "D" and "F" do not include mold flash or protrusions - Mold
flash or protrusions shall not exceed 0.15mm (.006inc.)
0016020 D
24/26
L6910G
Table 1. Revision History
Date
Revision
May 2005
1
Description of Changes
First Issue
25/26
L6910G
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners
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