Cherry CS5151GD16 Cpu 4-bit nonsynchronous buck controller Datasheet

CS5151
CS5151
CPU 4-Bit Nonsynchronous Buck Controller
Features
Description
The CS5151 is a 4-bit nonsynchronous N-Channel buck controller. It is designed to provide
unprecedented transient response
for today’s demanding high-density, high-speed logic. The regulator
operates using a proprietary control
method, which allows a 100ns
response time to load transients.
The CS5151 is designed to operate
over a 4.25-16V range (VCC) using
12V to power the IC and 5V as the
main supply for conversion.
The CS5151 is specifically designed
to power Pentium® processors with
MMX™ Technology and other high
performance core logic. It includes
the following features: on board,
4-bit DAC, short circuit protection,
1.0% output tolerance, VCC monitor,
and programmable soft start capability. The CS5151 is upwards compatible with the 5-bit CS5156, allowing the mother board designer the
capability of using either the
CS5151 or the CS5156 with no
change in layout. The CS5151 is
available in 16 pin surface mount
and DIP packages.
■ N-Channel Design
■ Excess of 1MHz Operation
■ 100ns Transient Response
■ 4-Bit DAC
■ Upward Compatible with
5-Bit CS5155/5156 and
Adjustable CS5120/5121
■ 30ns Gate Rise/Fall Times
■ 1% DAC Accuracy
■ 5V & 12V Operation
■ Remote Sense
■ Programmable Soft Start
■ Lossless Short Circuit
Protection
Application Diagram
■ VCC Monitor
Switching Power Supply for core logic - Pentium® processor with MMX™ Technology
12V
5V
VCC1 VCC2
VID0
VID0
VID1
VID2
VID1
VID2
VID3
VID3
2µH
2.1V to 3.5V @ 13A
Package Options
CS5151
3
MBR735
16 Lead SO Narrow & PDIP
1200µF/16V x 5
AlEl
COFF
PGnd
330pF
SS
LGnd
VFFB
VID0
VID1
VFB
1
COMP
LGnd
VID2
VFB
COMP
0.33µF
■ Overvoltage Protection
IRL3103
VGATE
1,2
0.1µF
■ Current Sharing
1200µF/16V x 3
AlEl
0.1µF
■ Adaptive Voltage
Positioning
■ V2™ Control Topology
VID3
3.3k
VCC1
NC
SS
100pF
NC
COFF
PGnd
VGATE
VFFB
VCC2
V2 is a trademark of Switch Power, Inc.
Pentium is a registered trademark and MMX is a trademark of Intel Corporation.
Cherry Semiconductor Corporation
2000 South County Trail, East Greenwich, RI 02818
Tel: (401)885-3600 Fax: (401)885-5786
Email: [email protected]
Web Site: www.cherry-semi.com
Rev. 1/5/99
1
A
®
Company
CS5151
Absolute Maximum Ratings
Pin Name
Max Operating Voltage
Max Current
VCC1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25mA DC/1.5A peak
VCC2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20mA DC/1.5A peak
SS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-100µA
COMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .200µA
VFB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA
COFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA
VFFB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA
VID0 - VID3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-50µA
VGATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100mA DC/1.5A peak
LGnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25mA
PGnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100mA DC/1.5A peak
Operating Junction Temperature, TJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .°0 to 150°C
Lead Temperature Soldering
Wave Solder (through hole styles only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 sec. max, 260°C peak
Reflow (SMD styles only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 sec. max above 183°C, 230°C peak
Storage Temperature Range, TS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-65° to 150°C
ESD Susceptibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2kV
Electrical Characteristics: 0°C < TA < +70°C; 0°C < TJ < +85°C; 8V < VCC1 < 14V; 5V < VCC2 < 14V; DAC Code: VID2 = VID1 =
VID0 = 1; VID3 = 0; CVGATE = 1nF; COFF = 330pF; CSS = 0.1µF, unless otherwise specified.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
■ Error Amplifier
VFB Bias Current
Open Loop Gain
Unity Gain Bandwidth
COMP SINK Current
COMP SOURCE Current
COMP CLAMP Current
COMP High Voltage
COMP Low Voltage
PSRR
VFB = 0V
1.25V < VCOMP < 4V; Note 1
Note 1
VCOMP = 1.5V; VFB = 3V; VSS > 2V
VCOMP = 1.2V; VFB = 2.7V; VSS = 5V
VCOMP = 0V; VFB = 2.7V
VFB = 2.7V; VSS = 5V
VFB =3V
8V < VCC1 < 14V @ 1kHz; Note 1
■ VCC1 Monitor
Start Threshold
Stop Threshold
Hysteresis
Output switching
Output not switching
Start-Stop
3.75
3.70
3.90
3.85
50
4.05
4.00
V
V
mV
VID0, VID1, VID2, VID3
VID0, VID1, VID2, VID3
1.00
25
4.85
1.25
50
5.00
2.40
100
5.15
1.0
V
kΩ
V
%
1.2315
2.1186
2.2176
2.3166
2.4156
2.5146
2.6136
2.7126
2.8116
2.9106
3.0096
1.2440
2.1400
2.2400
2.3400
2.4400
2.5400
2.6400
2.7400
2.8400
2.9400
3.0400
1.2564
2.1614
2.2624
2.3634
2.4644
2.5654
2.6664
2.7674
2.8684
2.9694
3.0704
■ DAC
Input Threshold
Input Pull Up Resistance
Pull Up Voltage
Accuracy
VID3 VID2
VID1
VID0
1
1
1
1
1
1
1
0
1
1
0
1
1
1
0
0
1
0
1
1
1
0
1
0
1
0
0
1
1
0
0
0
0
1
1
1
0
1
1
0
0
1
0
1
50
500
0.4
30
0.4
4.0
60
0.3
60
3000
2.5
50
1.0
4.3
160
85
Measure VFB = VCOMP, 25°C ≤ TJ ≤ 85°C
2
1.0
8.0
70
1.6
5.0
600
µA
dB
kH
mA
µA
mA
V
mV
dB
V
V
V
V
V
V
V
V
V
V
V
VID0 = 1; VID3 = 0; CVGATE = 1nF; COFF = 330pF; CSS = 0.1µF, unless otherwise specified.
PARAMETER
■
DAC: continued
VID3
VID2
VID1
0
1
0
0
0
1
0
0
1
0
0
0
0
0
0
TEST CONDITIONS
VID0
0
1
0
1
0
■ VGATE
Out SOURCE Sat at 100mA
Out SINK Sat at 100mA
Out Rise Time
Out Fall Time
Shoot-Through Current
VGATE Resistance
VGATE Schottky
TYP
MAX
UNIT
3.1086
3.2076
3.3066
3.4056
3.5046
3.1400
3.2400
3.3400
3.4400
3.5400
3.1714
3.2724
3.3734
3.4744
3.5754
V
V
V
V
V
1.2
1.0
30
30
V
V
ns
ns
mA
kΩ
mV
20
50
600
2.0
1.5
50
50
50
100
800
1.6
25
1.0
0.50
0.9
3.3
100
3.3
0.95
1.0
2.5
5.0
200
6.0
1.10
1.1
3.0
ms
ms
%
V
V
V
VFFB = 0 to 5V to VGATE = 9V to 1V;
VCC1 = VCC2 = 12V
VFFB = 0V
100
125
ns
■ Supply Current
ICC1
ICC2
Operating ICC1
Operating ICC2
No Switching
No Switching
VFB = COMP = VFFB
VFB = COMP = VFFB
8.5
1.6
8
2
13.5
3.0
13
5
mA
mA
mA
mA
■ COFF
Normal Charge Time
Extension Charge Time
Discharge Current
VFFB = 1.5V; VSS = 5V
VSS = VFFB = 0
COFF to 5V; VFB >1V
1.0
5.0
5.0
1.6
8.0
2.2
11.0
µs
µs
mA
VFB = VCOMP; VFFB = 2V;
Record VGATE Pulse High Duration
VFFB = 0V
10
30
50
µs
35
50
65
%
■ Soft Start (SS)
Charge Time
Pulse Period
Duty Cycle
COMP Clamp Voltage
VFFB SS Fault Disable
High Threshold
■ PWM Comparator
Transient Response
VFFB Bias Current
■ Time Out Timer
Time Out Time
Fault Mode Duty Cycle
Measure VCC2 – VGATE
Measure VGATE – VPGnd;
1V < VGATE < 9V; VCC1 = VCC2 = 12V
9V > VGATE > 1V; VCC1 = VCC2 = 12V
Note 1
Resistor to LGnd
LGnd to VGATE @ 10mA
MIN
(Charge Time/Pulse Period) × 100
VFB = 0V; VSS = 0
VGATE = Low
Note 1: Guaranteed by design, not 100% tested in production.
3
0.3
µA
CS5151
Electrical Characteristics: 0°C < TA < +70°C; 0°C < TJ < +85°C; 8V < VCC1 < 14V; 5V < VCC2 < 14V; DAC Code: VID2 = VID1 =
CS5151
Package Pin Description
PACKAGE PIN #
PIN SYMBOL
FUNCTION
16L SO Narrow & PDIP
1,2,3,4
VID0 – VID3
Voltage ID DAC input pins. These pins are internally pulled up to 5V
providing logic ones if left open. The DAC range is 2.14V to 3.54V with
100mV increments. VID0 - VID3 select the desired DAC output voltage.
Leaving all 4 DAC input pins open results in a DAC output voltage of
1.244V, allowing for adjustable output voltage, using a traditional resistor divider.
5
SS
Soft Start Pin. A capacitor from this pin to LGnd in conjunction with
internal 60µA current source provides soft start function for the controller. This pin disables fault detect function during Soft Start. When a
fault is detected, the soft start capacitor is slowly discharged by internal
2µA current source setting the time out before trying to restart the IC.
Charge/discharge current ratio of 30 sets the duty cycle for the IC when
the regulator output is shorted.
6, 12
NC
No connection.
7
COFF
A capacitor from this pin to ground sets the time duration for the on
board one shot, which is used for the constant off time architecture.
8
VFFB
Fast feedback connection to the PWM comparator. This pin is connected
to the regulator output. The inner feedback loop terminates on time.
9
VCC2
Boosted power for the gate driver.
10
VGATE
MOSFET driver pin capable of 1.5A peak switching current.
11
PGnd
High current ground for the IC. The MOSFET driver is referenced to this
pin. Input capacitor ground and the anode of the Schottky diode should
be tied to this pin.
13
VCC1
Input power for the IC.
14
LGnd
Signal ground for the IC. All control circuits are referenced to this pin.
15
COMP
Error amplifier compensation pin. A capacitor to ground should be provided externally to compensate the amplifier.
16
VFB
Error amplifier DC feedback input. This is the master voltage feedback
which sets the output voltage. This pin can be connected directly to the
output or a remote sense trace.
4
CS5151
Block Diagram
VCC2
VCC1
-
VCC1 Monitor
Comparator
5V
+
-
3.90V
3.85V
0.7V
+
2µA
Q
S
Q
PGnd
FAULT
FAULT
Latch
SS High
Comparator
-
VID0
VID2
R
FAULT
+
60µA
SS
VID1
VGATE
SS Low
Comparator
4 BIT
DAC
+
VID3
2.5V
Error
Amplifier
-
PWM
Comparator
-
VFB
Slow Feedback
COMP
VFFB
R
Maximum
On-Time
Timeout
+
S
Fast Feedback
Extended
Off-Time
Timeout
-
Off-Time
Timeout
GATE = OFF
Q
PWM
Latch
Normal
Off-Time
Timeout
GATE = ON
Q
COFF
One Shot
S
+
LGnd
COFF
R
Q
VFFB Low
Comparator
1V
PWM
COMP
Time Out
Timer
(30µs)
Edge Triggered
Applications Information
The V2™ control method is illustrated in Figure 1. The output voltage is used to generate both the error signal and the
ramp signal. Since the ramp signal is simply the output
voltage, it is affected by any change in the output regardless of the origin of that change. The ramp signal also contains the DC portion of the output voltage, which allows
the control circuit to drive the main switch to 0% or 100%
duty cycle as required.
A change in line voltage changes the current ramp in the
inductor, affecting the ramp signal, which causes the V2™
control scheme to compensate the duty cycle. Since the
change in inductor current modifies the ramp signal, as in
current mode control, the V2™ control scheme has the
same advantages in line transient response.
A change in load current will have an affect on the output
voltage, altering the ramp signal. A load step immediately
changes the state of the comparator output, which controls
the main switch. Load transient response is determined
only by the comparator response time and the transition
speed of the main switch. The reaction time to an output
load step has no relation to the crossover frequency of the
error signal loop, as in traditional control methods.
The error signal loop can have a low crossover frequency,
since transient response is handled by the ramp signal loop.
The main purpose of this ‘slow’ feedback loop is to provide
DC accuracy. Noise immunity is significantly improved,
since the error amplifier bandwidth can be rolled off at a low
frequency. Enhanced noise immunity improves remote sens-
Theory of Operation
V2™ Control Method
The V2™ method of control uses a ramp signal that is generated by the ESR of the output capacitors. This ramp is
proportional to the AC current through the main inductor
and is offset by the value of the DC output voltage. This
control scheme inherently compensates for variation in
either line or load conditions, since the ramp signal is generated from the output voltage itself. This control scheme
differs from traditional techniques such as voltage mode,
which generates an artificial ramp, and current mode,
which generates a ramp from inductor current.
PWM
Comparator
+
VGATE
C
–
Ramp
Signal
VFFB
VFB
Error
Amplifier
COMP
Error
Signal
Output
Voltage
Feedback
–
E
+
Reference
Voltage
Figure 1: V2™ Control Diagram
5
CS5151
Applications Information: continued
ing of the output voltage, since the noise associated with
long feedback traces can be effectively filtered.
Line and load regulation are drastically improved because
there are two independent voltage loops. A voltage mode
controller relies on a change in the error signal to compensate for a deviation in either line or load voltage. This
change in the error signal causes the output voltage to
change corresponding to the gain of the error amplifier,
which is normally specified as line and load regulation. A
current mode controller maintains fixed error signal under
deviation in the line voltage, since the slope of the ramp
signal changes, but still relies on a change in the error signal for a deviation in load. The V2™ method of control
maintains a fixed error signal for both line and load variation, since the ramp signal is affected by both line and load.
approximately equal to the maximum on time, resulting in
a 50% duty cycle. Then, the Gate pin will drive high, and
the cycle repeats.
When regulator output voltage achieves the 1V level present at the COMP pin, regulation has been achieved and
normal off time will ensue. The PWM comparator terminates the switch on time, with off time set by the COFF
capacitor. The V2™ control loop will adjust switch duty
cycle as required to ensure the regulator output voltage
tracks the output of the error amplifier.
The soft start and COMP capacitors will charge to their
final levels, providing a controlled turn on of the regulator
output. Regulator turn on time is determined by the COMP
capacitor charging to its final value. Its voltage is limited
by the soft start COMP clamp and the voltage on the soft
start pin (see Figures 2 and 3).
Constant Off Time
To maximize transient response, the CS5151 uses a constant off time method to control the rate of output pulses.
During normal operation, the off time of the high side
switch is terminated after a fixed period, set by the COFF
capacitor. To maintain regulation, the V2™ control loop
varies switch on time. The PWM comparator monitors the
output voltage ramp, and terminates the switch on time.
Constant off time provides a number of advantages. Switch
duty cycle can be adjusted from 0 to 100% on a pulse by
pulse basis when responding to transient conditions. Both
0% and 100% duty cycle operation can be maintained for
extended periods of time in response to load or line transients. PWM slope compensation to avoid sub-harmonic
oscillations at high duty cycles is avoided.
Switch on time is limited by an internal 30µs timer, minimizing stress to the power components.
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Trace 3 - 12V input (VCC1 and VCC2) (5V/div.)
Trace 4 - 5V Input (1V/div.)
Programmable Output
The CS5151 is designed to provide two methods for programming the output voltage of the power supply. A four
bit on board digital to analog converter (DAC) is used to
program the output voltage from 2.14V to 3.54V in 100mV
steps, depending on the digital input code. If all four bits
are left open, the CS5151 enters adjust mode. In adjust
mode, the designer can choose any output voltage by using
resistor divider feedback to the VFB and VFFB pins, as in traditional controllers. The CS5151 is specifically designed to
be upwards compatible with the CS5156, which uses a five
bit DAC code.
Figure 2: CS5151 demonstration board startup in response to increasing
12V and 5V input voltages. Extended off time is followed by normal off
time operation when output voltage achieves regulation to the error
amplifier output.
Start Up
Until the voltage on the VCC1 supply pin exceeds the 3.9V
monitor threshold, the soft start and gate pins are held low.
The FAULT latch is reset (no Fault condition). The output
of the error amplifier (COMP) is pulled up to 1V by the
comparator clamp. When the VCC1 pin exceeds the monitor
threshold, the Gate output is activated, and the soft start
capacitor begins charging. The Gate output will remain on,
enabling the NFET switch, until terminated by either the
PWM comparator, or the maximum on time timer.
If the maximum on time is exceeded before the regulator
output voltage achieves the 1V level, the pulse is terminated. The Gate pin drives low for the duration of the extended off time. This time is set by the time out timer and is
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - COMP Pin (error amplifier output) (1V/div.)
Trace 4 - Soft Start Pin (2V/div.)
Figure 3: CS5151 demonstration board startup waveforms.
6
If the input voltage rises quickly, or the regulator output is
enabled externally, output voltage will increase to the level
set by the error amplifier output more rapidly, usually
within a couple of cycles (see Figure 4).
Trace1 - Regulator Output Voltage (10V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 6: Peak-to-peak ripple on VOUT = 2.8V, IOUT = 13A (heavy load).
Trace 1 - Regulator Output Voltage (5V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Transient Response
The CS5151 V2™ control loop’s 100ns reaction time provides unprecedented transient response to changes in
input voltage or output current. Pulse by pulse adjustment
of duty cycle is provided to quickly ramp the inductor current to the required level. Since the inductor current cannot
be changed instantaneously, regulation is maintained by
the output capacitor(s) during the time required to slew the
inductor current.
Overall load transient response is further improved
through a feature called “adaptive voltage positioning”.
This technique pre-positions the output capacitor’s voltage
to reduce total output voltage excursions during changes
in load.
Holding tolerance to 1% allows the error amplifier’s reference voltage to be targeted +40mV high without compromising DC accuracy. A “droop resistor“, implemented
through a PC board trace, connects the error amplifier’s
feedback pin (VFB) to the output capacitors and load and
carries the output current. With no load, there is no DC
drop across this resistor, producing an output voltage
tracking the error amplifier’s, including the +40mV offset.
When the full load current is delivered, an 80mV drop is
developed across this resistor. This results in output voltage being offset -40mV low.
The result of adaptive voltage positioning is that additional
margin is provided for a load transient before reaching the
output voltage specification limits. When load current suddenly increases from its minimum level, the output capacitor is pre-positioned +40mV. Conversely, when load current suddenly decreases from its maximum level, the output capacitor is pre-positioned -40mV (see Figures 7, 8, and
9). For best transient response, a combination of a number
of high frequency and bulk output capacitors are usually
used.
Figure 4: CS5151 demonstration board enable startup waveforms.
Normal Operation
During normal operation, switch off time is constant and
set by the COFF capacitor. Switch on time is adjusted by the
V2™ control loop to maintain regulation. This results in
changes in regulator switching frequency, duty cycle, and
output ripple in response to changes in load and line.
Output voltage ripple will be determined by inductor ripple current working into the ESR of the output capacitors
(see Figures 5 and 6).
Trace 1 - Regulator Output Voltage (10V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Figure 5: Peak-to-peak ripple on VOUT = 2.8V, IOUT = 0.5A (light load).
7
CS5151
Applications Information: continued
CS5151
Applications Information: continued
If the maximum on time is exceeded while responding to a
sudden increase in load current, a normal off time occurs to
prevent saturation of the output inductor.
Trace1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (13 to 0.5 Amps) (20V/div.)
Figure 9: CS5151 demonstration board response to 13A load turn off
(output set for 2.8V). V2™ control topology immediately connects
inductor to ground, providing 0% duty cycle. Regulation is achieved in
less than 10µs.
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - Regulator Output Current (20V/div.)
Figure 7: CS5151 demonstration board response to a 0.5 to 13A load
pulse (output set for 2.8V).
Protection and Monitoring Features
VCC1 Monitor
To maintain predictable startup and shutdown characteristics an internal VCC1 monitor circuit is used to prevent the
part from operating below 3.75V minimum startup. The
VCC1 monitor comparator provides hysteresis and guarantees a 3.70V minimum shutdown threshold.
Short Circuit Protection
A lossless hiccup mode short circuit protection feature is
provided, requiring only the soft start capacitor to implement. If a short circuit condition occurs (VFFB < 1V), the VFFB
low comparator sets the FAULT latch. This causes the MOSFET to shut off, disconnecting the regulator from its input
voltage. The soft start capacitor is then slowly discharged by
a 2µA current source until it reaches its lower 0.7V threshold.
The regulator will then attempt to restart normally, operating
in its extended off time mode with a 50% duty cycle, while
the soft start capacitor is charged with a 60µA charge current.
If the short circuit condition persists, the regulator output
will not achieve the 1V low VFFB comparator threshold
before the soft start capacitor is charged to its upper 2.5V
threshold. If this happens the cycle will repeat itself until the
short is removed. The soft start charge/discharge current
ratio sets the duty cycle for the pulses (2µA/60µA = 3.3%),
while actual duty cycle is half that due to the extended off
time mode (1.65%).
This protection feature results in less stress to the regulator
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (0.5 to 13 Amps) (20V/div.)
Figure 8: CS5151 demonstration board response to 13A load turn on
(output set for 2.8V). Upon completing a normal off time, the V2™ control loop immediately connects the inductor to the input voltage, providing 100% duty cycle. Regulation is achieved in less than 20µs.
8
CS5151
Applications Information: continued
components, input power supply, and PC board traces
than occurs with constant current limit protection (see
Figures 10 and 11).
If the short circuit condition is removed, output voltage
will rise above the 1V level, preventing the FAULT latch
from being set, allowing normal operation to resume.
External Output Enable Circuit
On/off control of the regulator can be implemented
through two additional discrete components (see Figure 12).
This circuit operates by pulling the soft start pin high, and
the VFFB pin low, emulating a short circuit condition.
5V
MMUN2111T1 (SOT-23)
5
SS
CS5151
8 V
FFB
IN4148
Shutdown
Input
Figure 12: Implementing shutdown with the CS5151.
Trace 4 - 5V Supply Voltage (2V/div.)
Trace 3 - Soft Start Timing Capacitor (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
External Power Good Circuit
An optional Power Good signal can be generated through
the use of four additional external components (see Figure
15). The threshold voltage of the Power Good signal can be
adjusted per the following equation:
Figure 10: CS5151 demonstration board hiccup mode short circuit protection. Gate pulses are delivered while the soft start capacitor charges,
and cease during discharge.
VPower Good =
(R1 + R2) × 0.65V
R2
This circuit provides an open collector output that drives
the Power Good output to ground for regulator voltages
less than VPower Good.
5V
R3
10k
R1
10k
PN3904
VOUT
CS5151
Power Good
PN3904
R2
6.2k
Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 2 = Inductor Switching Node (2V/div.)
Figure 11: Startup with regulator output shorted.
Figure 13: Implementing Power Good with the CS5151.
Overvoltage Protection
Overvoltage protection (OVP) is provided as result of the
normal operation of the V2™ control topology and requires
no additional external components. The control loop
responds to an overvoltage condition within 100ns, causing
the MOSFET to shut off, disconnecting the regulator from
its input voltage.
9
CS5151
Applications Information: continued
Channel 3 = VGATE
M1= VGATE - 5VIN
Channel 2 = Inductor Switching Node
Trace 3 = 12V Input (VCC1) and VCC2) (10V/div.)
Trace 4 = 5V Input (2V/div.)
Trace 1 = Regulator Output Voltage (1V/div.)
Trace 2 = Power Good Signal (2V/div.)
Figure 15: CS5151 gate drive waveforms depicting rail to rail swing.
Figure 14: CS5151 demonstration board during power up. Power Good
signal is activated when output voltage reaches 1.70V.
The most important aspect of MOSFET performance is
RDSON, which effects regulator efficiency and MOSFET
thermal management requirements.
The power dissipated by the MOSFET and the Schottky
diode may be estimated as follows;
Switching MOSFET:
Power = ILOAD2 × RDSON × duty cycle
Selecting External Components
The CS5151 can be used with a wide range of external
power components to optimize the cost and performance of
a particular design. The following information can be used
as general guidelines to assist in their selection.
NFET Power Transistors
Both logic level and standard MOSFETs can be used. The
reference designs derive gate drive from the 12V supply
which is generally available in most computer systems and
use logic level MOSFETs. A charge pump may be easily
implemented to support 5V only systems. Multiple
MOSFETs may be paralleled to reduce losses and improve
efficiency and thermal management.
Voltage applied to the MOSFET gate depends on the application circuit used. The gate driver output is specified to
drive to within 1.5V of ground when in the low state and to
within 2V of its bias supply when in the high state. In practice, the MOSFET gate will be driven rail to rail due to
overshoot caused by the capacitive load it presents to the
controller IC. For the typical application where VCC1 = VCC2
= 12V and 5V is used as the source for the regulator output
current, the following gate drive is provided;
Schottky diode:
Power = VFORWARD × ILOAD × (1 - duty cycle)
VOUT + VFORWARD
Duty Cycle = V + V
IN
FORWARD - (ILOAD × RDSON OF SWITCH FET)
Off Time Capacitor (COFF)
The COFF timing capacitor sets the regulator off time:
TOFF = COFF × 4848.5
When the VFFB pin is less than 1V, the current charging the
COFF capacitor is reduced. The extended off time can be calculated as follows:
TOFF = COFF × 24,242.5.
Off time will be determined by either the TOFF time, or the
time out timer, whichever is longer.
The preceding equations for duty cycle can also be used to
calculate the regulator switching frequency and select the
VGATE = 12V - 5V = 7V (see Figure 15).
10
COFF timing capacitor:
COFF =
Thermal Impedance =
Period × (1 - duty cycle)
,
4848.5
A heatsink may be added to TO-220 components to reduce
their thermal impedance. A number of PC board layout
techniques such as thermal vias and additional copper foil
area can be used to improve the power handling capability
of surface mount components.
where:
Period =
1
switching frequency
EMI Management
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance with EMI/EMC regulations, additional components may be added to reduce noise emissions. These
components are not required for regulator operation and
experimental results may allow them to be eliminated. The
input filter inductor may not be required because bulk filter
and bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the circuit board and input power supply. Placement of the
power component to minimize routing distance will also
help to reduce emissions.
“Droop” Resistor for Adaptive Voltage Positioning
Adaptive voltage positioning is used to reduce output voltage excursions during abrupt changes in load current.
Regulator output voltage is offset +40mV when the regulator is unloaded, and -40mV at full load. This results in
increased margin before encountering minimum and maximum transient voltage limits, allowing use of less capacitance on the regulator output (see Figure 7).
To implement adaptive voltage positioning, a “droop”
resistor must be connected between the output inductor
and output capacitors and load. This is normally implemented by a PC board trace of the following value:
RDROOP =
TJUNCTION(MAX) - TAMBIENT
Power
80mV
IMAX
Adaptive voltage positioning can be disabled for improved
DC regulation by connecting the VFB pin directly to the load
using a separate, non-load current carrying circuit trace.
2µH
Input and Output Capacitors
These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to provide acceptable ripple on the input supply lines and regulator output voltage. Key specifications for input capacitors
are their ripple rating, while ESR is important for output
capacitors. For best transient response, a combination of
low value/high frequency and bulk capacitors placed close
to the load will be required.
33Ω
2µH
1200µF x 3/16V
+
1000pF
Figure 16: Filter components
Figure 17: Input Filter
Layout Guidelines
1. Place 12V filter capacitor next to the IC and connect
capacitor ground to pin 11 (PGnd).
2. Connect pin 11 (PGnd) with a separate trace to the
ground terminals of the 5V input capacitors.
3. Place fast feedback filter capacitor next to pin 8 (VFFB)
and connect its ground terminal with a separate, wide trace
directly to pin 14 (LGnd).
4. Connect the ground terminals of the Compensation
capacitor directly to the ground of the fast feedback filter
capacitor to prevent common mode noise from effecting
the PWM comparator.
5. Place the output filter capacitor(s) as close to the load as
possible and connect the ground terminal to pin 14 (LGnd).
Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade
transient response.
Thermal Management
Thermal Considerations for Power MOSFETs and Diodes
In order to maintain good reliability, the junction temperature of the semiconductor components should be kept to a
maximum of 150°C or lower. The thermal impedance (junction to ambient) required to meet this requirement can be
calculated as follows:
6. To implement adaptive voltage positioning, connect
both slow and fast feedback pins 16 (VFB) and 8 (VFFB) to
the regulator output right at the inductor terminal. Connect
inductor to the output capacitors via a trace with the following resistance:
11
CS5151
Applications Information: continued
CS5151
Applications Information: continued
RTRACE =
80mV
IMAX
This causes the output voltage to be +40mV with no load,
and -40mV with a full load, improving regulator transient
response. This trace must be wide enough to carry the full
output current. (Typical trace is 1.0 inch long, 0.17 inch
wide). Care should be taken to minimize any additional
losses after the feedback connection point to maximize regulation.
7. If DC regulation is to be optimized (at the expense of
degraded transient regulation), adaptive voltage positioning can be disabled by connecting to VFB pin directly to the
load with a separate trace (remote sense).
8. Place 5V input capacitors close to the switching MOSFET.
Route gate drive signal VGATE (pin 10) with a trace that is a
minimum of 0.025 inches wide.
VCC
0.1µF
To the negative terminal of the
input capacitors
15
11
1.0µF
VCOMP
8
100pF
VFFB
5
SOFTSTART
OFF TIME
To the negative terminal of the output capacitors
Figure 18: Layout Guidelines
12
CS5151
Additional Application Circuits
5V
3.3V
12V
0.1µF
MBRS
120
1µF
MBRS120
+
1µF
MBRS120
+
100µF/10V x 3
Tantalum
Si9410
Si4410DY
1µF
VCC2
VCC1
VCC1
VGATE
3µH
3.3V/10A
VID1
VID2
2
CS5151
COFF
SS
0.1µF
3.3k
0.33µF
+
100µF/10V x 3
Tantalum
5V
MBRS120
+
1µF
100µF/10V x 3
Tantalum
Si4410
VCC2
VGATE
3µH
Remote
Sense
3.3V/10A
VID0
VID1
VID2
VID3
VFB
10Ω
CS5151
2
+
100µF/10V x 3
Tantalum
MBR1535CT
COFF
1,3
330pF
SS
0.1µF
PGnd
COMP
LGnd
0.33µF
VFFB
VFFB
3.3k
100pF
Figure 21: 3.3V to 2.5V/7A converter with 12V bias.
Figure 19: 5V to 3.3V/10A converter.
VCC1
PGnd
COMP
LGnd
100pF
MBRS120
1,3
SS
VFFB
3.3k
100pF
2.5V/7A
+
MBR1535CT
COFF
0.33µF
1µF
VFB
330pF
VFB
LGnd
0.1µF
CS5151
2
PGnd
COMP
MBRS
120
5µH
1,3
330pF
0.1µF
VGATE
VID3
MBR1535CT
VID3
VCC2
VID0
VID0
VID1
VID2
33µF/25V x 3
Tantalum
Connect to
other circuits for
current sharing
Figure 20: 5V to 3.3V/10A converter with current sharing.
13
100µF/10V x 2
Tantalum
CS5151
Package Specification
PACKAGE THERMAL DATA
PACKAGE DIMENSIONS IN mm (INCHES)
D
Lead Count
Metric
Max
Min
10.00
9.80
19.69
18.67
16L SO Narrow
16L PDIP
Thermal Data
English
Max Min
.394 .386
.775 .735
RΘJC
RΘJA
16L
SO Narrow
28
115
typ
typ
16L
PDIP
42
80
˚C/W
˚C/W
Surface Mount Narrow Body (D); 150 mil wide
4.00 (.157)
3.80 (.150)
6.20 (.244)
5.80 (.228)
0.51 (.020)
0.33 (.013)
1.27 (.050) BSC
1.75 (.069) MAX
1.57 (.062)
1.37 (.054)
1.27 (.050)
0.40 (.016)
0.25 (.010)
0.19 (.008)
D
0.25 (0.10)
0.10 (.004)
REF: JEDEC MS-012
Plastic DIP (N); 300 mil wide
7.11 (.280)
6.10 (.240)
8.26 (.325)
7.62 (.300)
1.77 (.070)
1.14 (.045)
2.54 (.100) BSC
3.68 (.145)
2.92 (.115)
.356 (.014)
.203 (.008)
0.39 (.015)
MIN.
.558 (.022)
.356 (.014)
D
REF: JEDEC MS-001
Some 8 and 16 lead
packages may have
1/2 lead at the end
of the package.
All specs are the same.
Ordering Information
Part Number
CS5151GD16
CS5151GDR16
CS5151GN16
Rev. 1/5/99
Description
16L SO Narrow
16L SO Narrow (tape & reel)
16L PDIP
Cherry Semiconductor Corporation reserves the right to
make changes to the specifications without notice. Please
contact Cherry Semiconductor Corporation for the latest
available information.
14
© 1999 Cherry Semiconductor Corporation